I can absolutely murder on the AA gun, I got all the requirements for the stationary weapon medal in one and a half games with it. The biggest problem is getting sniped off of it, but on some maps if you have a good team holding certain flags, you can stay relatively safe.
My son is a decent pilot (i cant seem to fly at all in this game) so I rack up a lot of kills gunning for him. His main gun and mine simultaneously firing on a smaller plane can shred it in no time, bombers are pretty tough and take more effort to bring down. Its also pretty easy to kill people off the AA guns from the gunner seat too, I know where they are and they cant run fast enough. hehe