Good. Way better than the constant tweaking and repairing that characterized AEGs. You get used to the tank and hose and the increase in range, ROF, accuracy and reliability makes all the difference. Expect to drop about $750-1000 on the rig.
Half my team in Indiana run G&P guns with P* internals. When they are up and running, they are great. Mostly the same things that OldSchool said, increased range and the ROF (even on semi) is insane. But IMO, the air rig setup can make the system about as finicky or reliable as an AEG. Both require attention to run well and occasionally crap out on you anyways.
I'm sticking with my G&P AEGs for the foreseeable future. I don't think the improved range is enough to warrant the extra weight and cost.