My interest in Airsoft dwindled fast after just getting started, but this thing is re-ignighting it:
Tippmann M4 Carbine
Full Overview
CO2 is way cheaper than a PolarStar HPA(high-pressure air) setup, and you can get CO2 fillups for around $5 bucks. (CO2 tank and hose hookups are around $50, MUCH cheaper than HPA)
Tippmann M4 are supposed to be incredibly fun to shoot with very strong recoil, and incredible reliability/performance. You can adjust the FPS and rate-of-fire with a simple allen screw, and there are ZERO electrics on the gun to fuck up or break--compared to polarstar.
You can convert it HPA as well, but then of course a compressed air tank and remote are very expensive, though you gain the big advantage of using your high-cap AEG mags. I like also that you can use individiual clips with a 12-gram CO2 cartridge.