In the last couple of games i came to the conclusion that the drone skill is way too strong. The damage is incredible high (as well as the creeping is if you use the skill) and the heal is quite strong too. I'll post a replay from an intern game where you can see how imba the skill is.
It's very strong to be sure, but it requires quite a bit of luck and skill to use properly as an offensive measure. Mainly, you got to stay in your drone range and make sure the target player is one of the only targets remaining. Furthermore, you must not be at full health, because it seems to me the drone AI try to heal you, and not hurt things around, and thus will not attack if you dont need healing. This is usually not a problem, just an observation. The healing effect is not instantaneous as well, as I usually get healed only when the timer ends and every drone which has drained HP is on a return to base pattern. On these situations however, if nearly all your drones drained some hp, the healing amount is nearly 2500 HP, which is very, very high. But it is not as reliable as a healing kit, and those heal for about the same amount, instantly (higher mana cost though). It is unreliable in the heat of battle, it mainly serve as an aggravating attack, good creeper and post-battle healing.
I'll watch the replay closely, then give other observations later on.
Update: Well, I saw the game and I say the guy using the drone skill knew exactly how to use it. As for being IMBA though, I don't agree. It is very akin a repair kit.
Well i just found a bug which may be the reason why i thought it would be imba. Whenever you cast healing drones and they come back to you they count as killed creeps. So if check your -stats you might see 650 and your opponent got only 350, which looks like you creep way better, but you don't becouse the 300 are mostly your own healing drones. How well someone with healing drones really creeps is quite hard to say, so we should keep an eye on it.