I'd like to add my voice to the "GBBR just feel better" brigade. I spent a lot of time (and money :P) upgrading AEGs for two years before trying a GBBR once and immediately being converted.
I had upgraded my ICS m4 to try to decrease the delay between pulling the trigger and the shot leaving the barrel, and it was as fast as anything else I'd heard including Systema PTWs that I encountered but as an overall shooting experience the GBBR was just plain better.
I would say though that I was always a more effective player with my AEG that with my GBBR. I could carry three times the ammunition in my midcaps than I could in my GBBR mags. Airsoft isn't the most accurate projectile system and so by laws of averages the more aimed rounds I put downrange the more likely I was to hit the target. With an AEG I could triple-tap where I would single or double-tap without worrying about ammo.