So, I find myself with an hour or so to kill and nothing urgent to do, so let me pontificate a bit about gun drones.
In past editions, I found the vehicular gun drones to be the absolute best thing about Tau vehicles. In fourth edition (back when the Tau were incredibly good by even modern standards), they were basically free harassment units with the potential to cause significant problems for the enemy. Even in the era of kill points, the fact that the little two-drone units could contest objectives was absolutely golden. Today? Well, today they are arguably one of the most useful parts of our army.
Never Pay For Gun Drones
For many years this was my motto. I've never been terribly impressed with Gun Drone squadrons (I'm still not, honestly, though they are much better these days) and, with the possible exception of Stealth Suits, there isn't much motivation to attach them to other squads. You can get plentiful pulse fire from elsewhere, pinning isn't much of a motivation for the points, and the large unit footprint of a full squad makes the jetpack somewhat less than useful if it's your plan to keep it alive.
All of that changes, though, when dealing with vehicular drones. Since you aren't investing anything to field them (you want the devilfish anyway and Piranhas are worth it just as a cheap-as-chips fast fusion blaster) and since the enemy has no impetus to kill them (they aren't worth even First Blood points), their target priority is extremely low. Since you didn't really pay for them to begin with, you have no real need to keep them alive. They are basically free, independently operating guns that can fly around and do whatever seems like it would be disruptive for quite literally no risk to you. If they achieve anything at all, the will have earned more than their points back.
Their Advantages
1) Mobility: The Jet pack as well as the vehicle mount means you can get the drones where they need to be by turn 2. With Piranha, they can even assault things from the transport.
2) Firepower: While they don't pack an enormous punch, the fact that a two-drone squad packs firepower slightly better than a Heavy Bolter is important. It is just enough firepower to threaten most targets. Gun drones are unlikely to wipe units out unless used en masse, but they are very likely to damage units. They can take off the last hull point of a vehicle (assuming rear armor 10), they can knock a wound or two off most infantry squads, they have a chance to wound most MCs, they are twin-linked (giving them a better than average chance against flyers), they have Supporting Fire, and every once in a while they'll even pin somebody. When you are plinking away with little shots like this from 4-5 squads of the little guys, totally 10-12 drones altogether (without taking a slot or costing a point to field), suddenly they don't become funny anymore.
3) Surprisingly Durable: They aren't tough, but they are as tough as an Ork 'Ard Boy but with a jetpack. While nothing to write home about, when combined with the fact that they cost essentially nothing and are worth nothing, this is actually fairly substantial. Your enemy doesn't want to shoot at them, and the odd Rhino-mounted stormbolter won't be sufficient to make them go away. To kill them, the enemy needs to task an *actual unit* to shoot at them or assault them, and this is a colossal waste of his time and resources when we consider that you've still brought a ton of real Tau units to the party, too.
How to Use Them
In brief, Vehicular Gun Drones are an easy way to flood the board with harassment units. If playing a Mech Tau list, you are going to have 3-ish Devilfish and (I would recommend) 2-4 Piranha anyway. This means 10-14 gun drones floating around, causing trouble. Very few opponents can afford to deal with them, and most opponents won't bother. However, the incremental damage the drones do adds up over the course of a battle. Besides just shooting whatever seems opportune, here are some specialized uses for them:
1) Inhibit Movement: Clever drone usage can quickly make the enemy movement phase inconvenient. You can stick your drones places the enemy wants to move easily, and this forces them either to change their movement plans *or* engage the drones, and either option is a win for you. They can be particularly useful in getting in the way of flyers, again using their superior mobility to interfere with flight plans. If you bring lots and lots of drones, you can make a virtual movement minefield for deepstrikers or any infantry unit trying to get by to assault your main force.
2) Assault!: I realize it sounds crazy, but Drones are very capable of assaulting certain (weakened) units and keeping them tied up for 2 turns. Things like IG Heavy Weapons Squads or Eldar Rangers or the like aren't super likely to win the first round of combat by more than 1, which leaves the drones a slightly less than 50% chance of staying for that second round and screwing up a round of shooting, and that's assuming they don't manage to kill anybody themselves (which, while not likely, is entirely possible against squishy support units). This can frequently be worth the gamble, especially if you can get a couple teams of drones to assault all at once. I once tied up a small squad of Wraithguard this way for a turn or two, which paid off in a big way.
3) Anti-aircraft: I mentioned this above, but it bears repeating: Tau drones are actually pretty good anti-aircraft units. They have a decent rate of fire, twin-linking, and enough strength to pen AV10. Often this is all that is needed to knock down a flyer, and constitutes a minimal investment on your part.
The trick with using vehicular drones is getting them off their transports as soon as possible. Use the ride to boost them forward for a turn (by all means remember that the cargo of the devilfish is more important than the drones) and then pop them off to go their own way before their ride moves out next turn. If they go down with the transport, they may as well never have existed. They are best used as small independent units and the sooner you detach them, often the better off you'll be. They really do drive your opponents crazy, and I can't recommend them enough. Naturally, however, this presupposes you'll be taking a good number of Tau vehicles anyway (Devilfish especially), but this is typically the case for most Tau armies I've seen--2-3 devilfish are common, and 4-6 drones can still cause plenty of headaches.
Anyway, I hope you found this interesting, and I'd love to hear what you guys have found for those free drones to do.