I absolutly loved the demo of this game, and will be buying it asap! I do agree it's a bit weird not being able to lie down, but I guess that's to stop snipers from completly dominating the game. On the Xbox, helicopter controles are absolutly retarded and don't make a sceric of sense, but I'll assume that in the full game you can choose the configuration. vehicles as far as I've noticed are very well balanced, providing a challenge to overcome with teamwork, but never to the point of it seeming cheap and OP. Even the UAV's which have a devestating payload for such a small, high altitude vehicle, can be taken out by a good sniper with a few shots, and likewise, a good pilot can evade destruction just as easily.
Only thing that seems to me a little bit unbalanced is the tank machine gun. It's accuracy is pin point, it has quite a long range (with a zoom function) and being on a tank, is extremely difficult to neutralize. I was in a squad with 3 friends and we were all dedicated to the function and maintenence of this one tank for a whole round, me being on the gun, one driving, and 2 for repairs in the rear. We never died, and I got a total of 53 kills, 3 helicopters and 2 UAVs, just using the machine gun. Friend driving/cannoning killed 5 enemy tanks, and we never had issue because the two engineers hiding behind us tandem repaired.
I wouldn't really call this overpowered though, because unless you're with an organised group of friends this would NEVER happen. More often, people will drive their faces into the very center of action and be blown up instantly.
All in all, brilliant game, and I can't wait to get the full copy.