-Deep Striking: Well maybe this is not always a good thing but it can come in usefull especially for static armies who really lack in mobility. Besides if you are going to lose a unit deep striking the drones arent a bad loss. This can prove very effective when used right and is a bonus to the rest of the drones attributes.

Disadvantages
  -Leadership: These guys are going to run sometimes... assuming any live to run. With a mediocer starting leadership and a utterly poor leadership of 4 when they drop below 4 models they arent going to be turning around anytime soon.
  -Coste: Alright I personally do not think this is a problem but apparently the 12 points coste for one of these guys is a lot for some people. However considering that these are usually throw away units the 12 points makes you pause. Kroot are used more often as meat shields though I think this designation for drones is incorrect.
  -Volume of shots: This is the other point of their shooting, both an advantage and disadvantage in its own right. They each only get a single shot and with a BS of 2 even the twin linked only averages a BS of 3. This is not too great when you have things like mounted Firewarrior squads with the capability of putting out 24 shots, the 8 of a full drone squadron does not seem so great.

Those are just the general things, there are more specific things on both sides but these are the main issues that I can see. So next we have to ask ourselves the following:

What are the roles of Drones in the army?
When to use Drones?
How many Drones to use?

Alright looking at the first question. Drones form several support roles in the Tau army. First they act as shields for units. When in a mechanized or highly mobile list they usually form the front of a fighting wing of battlesuits, forcing leadership roles to shoot at the more juicy crisis and stealth suits. Aswell as adding numbers to the devision. In a static army I see drones most often used as shields for Firewarriors. Held in reserve behind the static squads and then sent forward to intercept the enemy before they reach the main Tau lines. These are just the two most generic roles, I also see them used as mobile scoring units, deep striking for shots on rear armor, flanking roles and much more and they can be used for this but I personally do not think this is what they should be used for. Such things tend to place too much importance on a unit that cannot be relied to stand up under fire and keep from running away. So let me go more in depth into the 3 differant general army list types and the drones place in such lists.

Mechanized: In mechanized I myself love to use drones in conjunction with the tanks, or at least buy them with the tanks. Pairs of gun drones are more potent for me than full squadrons. Frequently in shooting and assault 2 model units seem to do just as good or better than 8 model ones. Plus it forces your opponent to split your fire. I find that using the pair of gun drones upgrade for a hammerhead is an exelent way to fill in points and plug gaps in the army. Simply deploy them seperately from the tank at the start and there is no problem, they count as passengers so can do this. In addition to the large numbers of tiny drone squadrons in mech lists drones can be bought in squadrons to form up with crisis suits and this is also a very good option to take. Its much better to have the enemy kill a 12 point drone than a 40+ point suite.

Static: The drones can do several tasks in a static list than they can in a mechanized or hybrid list. The main one being in conjunction with static firewarrior squads. Formed up in front or behind the squads to await incoming opponents the drones have a chance of pinning the enemy in place for the Firewarriors to shoot up and can draw fire aswell as rush into assault with their wonderfull Initiative of 4 to lock the enemy up for a turn or two for the Firewarriors to reposition for a finishing blow when the combat ends. This way the Firebase stays relatively safe and the Firewarriors have easy and close by support. This also means that infiltrators and deep striking enemies can be countered easily, even if the Firewarriors get locked up the drones can give them a doorway out of combat to exact their vengeance latter on their own terms... in shooting of course. Holding up termies for a turn or two while the Firewarriors move back and the heavies come to the rescue can mean the differance between victory and defeat. Another purpose is objective claiming. Firewarriors in a static list are not going to be doing much of this which leaves it on the shoulders of the battlesuits and tanks, but who else to aid these highly mobile units than the good old reliable drones. Contesting objectives and seizing loot counters early the drones can do this quite well. Holding them in reserve to deep strike and claim quarters late in the game can really turn the tide sometimes. A 4 drone squadron will only set you back 48 points and with deep striking they can contest your opponents quarters without commiting your force to an attack, for static armies this can be vital. This is one of the few times I advise deep striking any Tau unit really.

Hybrid: Any and all drone roles can be used in a hybrid list but it mostly depends on what the list is going to be doing. Normally I find that drones are exelent for plugging holes and filling in gaps with hybrid lists. They can form a nice rogue element to confuse the opponent and leave them unsure as to your general plan. Whether deep striking them, forming them with battlesuits or simply having them provide aid to Infiltrating Kroot or mobile Firewarriors they can do a little of everything in a Hybrid list, they just should never, ever be counted on as a vital part of the plan... ever.

So a summary:
Mechanized- They can provide much needed numbers and support for advancing battlesuite teams and can be an absolute pest to the enemy in small squads of 2. They fill the holes and lend easy support to the various units in the army without losing much in the way of points or FOC slots.
Static- Table quarter contesting, Firewarrior shields, advance support for mobile elements the Drones give much needed mobility to static armies and allow them to contest the board early or late in the game.
Hybrid- A little of everything they are the rogue element, they are the man of all trades and lend support where its needed and when its needed while being cheap and expendable.

Now onto the duel questions of when to use drones and how many to use. These go together so I will answer them together. I rarely if ever use more than 10 drones. There is a point when drones cease to make up their value and become a detriment, I do not suggest taking more than 16 of them at any time, at least on a single FOC. There are also times where you will use more and times where you will use less. I use a fair amount of drones when I have to go on the attack. I know cover will not always be available and I would much rather lose a few drones than a few crisis suits and so I bring along 6-10 Drones to back up my spearhead of battlesuits. There are also times like in small games where a single pair of gun drones from a tank to protect my commander is more than enough. To determine this you have to determine a) what you are trying to do in the battle and b) what your opponent is trying to do. If you know all the fighting is going to be in a single quarter of the board, bringing 2 or 3 squadrons of 4 gun drones can be a great way to contest or capture quarters throughout the game and make sure that your opponent is not going to lock you down in one place and cut all your options. Then there are times when you know you simply have to hold out at all costes like in the mission of that name and spending points on drones is not a good idea. Keeping around 2 or 3 full squadrons to throw in when needed is a very good idea because no matter how long I make you read I cannot cover all of the situations for using and not using drones. Just as long as you know that they are not going to survive to concentrated fire, should never be the center of a strategy and should not be taken in too large of numbers you should do fine.

Now a short thing on shield drones. They are only available as wargear and have no guns but they confer a 4+ invulnerable save. The only time I see them as usefull under the current codex is as attatchments to broadside battlesuits, no other unit in the Tau army should really be taking these. The only other time I forsee them being usefull is on a commander who has 2 bodyguards who you do not want to have to pay 20 points for them to have shield gennerators, give your commander one and buy 2 shield drones, being in the majority you can take any low ap insta kill shots on them first. However with IC status few commanders are even going to have bodyguards but if you really do want them for fluff reasons than the shield drones may be usefull here.

Alright what you should take from this as I know I have done a bit of rambling. Firstly that drones are never really going to be the stars of the show. Still this is 40k and flukes do happen and those gun drones may take down a leman russ from behind but in general one simply cannot relly on them too heavily. This does not however make them useless. Used as support units to take damage away from other units in your army and to plug holes points wise they work extraordinarily well. For what you pay you get really rather a lot and this should not be ignored. They have weaknesses and some big ones but as long as you know their limits they will work well and can really throw a wrench into your opponents plan which is something I think we all want to do, I know I take particular relish in it. Lastly you should really not take a whole lot of them. A few drones is great, too many and they become dead weight. A balance is required and this will differ with differant people and lists. Though generally I find 10 a good mix,16 is the high end and you can have as few as you want. 

In conclusion just remember that Gun Drones though not the best thing in the world can be quite usefull if used right.

In inspiration from the Firewarrior Tactica thread a tid bit of fluff:

Shas'O Ximoro'An walked the battlefield examining the destruction the day had wrought. Bodies littered the battlefield though he took some satisfaction in seeing a far higher density of gue'la dead than Tau. Something crunched under the foot of his battlesuite. Looking down he spotted the ruined dome of a gun drone sticking out of the mud. Extending the digits of his hands he reached down and pried the thing from the earth. It bore numerous dents and scratches but the felling shot had been from an armor piercing grenade which had crashed right through the small machine blasting half of it away. Vaguely he remembered the drone detonating infront of him during the battle, saving him from a rather gruesome fate. Without thinking he brushed the front panel of his suite reasured that it was intact. That would be the last time he underestimated the value of the tiny machine. Dropping it back to the earth he headed off to finish his inspection. The orange glow of the setting sun glinted off the tiny shattered dome lying in the mud, the hero of the day.

Thats it folkes if you have read through to here I applaud you. Thanks for reading I hope you enjoyed it and find this of some use.