If you suck in mobile AA, then maybe it is you that sucks. My 25hours in AA and a 5.6KD with it says so.
All you gota do is wait for the flares to go away, relock on and stick 4 missiles in their face.
Well, for choppers anyway. It isnt gona work against jets that well - so you either catch them when their flares are reloading, or when they are dive bombing you - fill em full of cannon and put missiles in them. I have yet to come across a pilot who has flared my missiles when hes on a bombing run at me. I often take him out for about half my armour being depleted from the cannon it fires.
Keep an eye on the radar and UAV/spots picking up enemy jets and choppers. Face in their general direction and wait for your lock. If there are none on the radar, keep your turret spinning so you keep a good eye on the skies ready to take the bastards out.
Stinger sites do suck ass, I admit. They are only useful on Ghost Town - its the only map where they really pose a threat.
If you want AA to be ever so dominant, play Road Rage. AA absolutly kicks the shit out of A10's. You can take them out with the cannon and no missiles with a bit of skill and judgement.
What annoys me the most, is that this game is set in the modern battlefield (read the back of the BF2 case in case you hadnt realised this ingame by now). The moden battlefield has SAM sites, AA missiles and lockons. Choppers and jets have flares/chaff to confuse and divert missiles away from them. You dont see flak cannons shooting down planes in the modern battlefield...thats WW2 era. I suppose you may have a few odd ones here and there, but its mostly SAM missiles.
So why the fuck would a game based on the "modern battlefield" even be designed with flak cannons at all?
Stop whining and learn to use it properly.