I have the GNX3, and let me tell you, you can really create alot of unique sounds once you learn to mess with all of the effects. you can layer on a ton of effects at once, and the amp heads/cabs can color your sound in different ways.
however, even with that said, I barely ever use it. the distortion to me sounds too fuzzy. sometimes thats good if you want a good fuzz sound, but if you want a nice tight distortion, you may want an external EQ and compression. the mod effects are pretty good and believeable, and most are pretty cool sounding. the delay is okay. same with wah. its pretty believeable. not really thick and in your face if thats what you want- I dont think it sounds good that way anyways.
if you like analog effect sounds, this isnt for you. its very digital, but the pure mass of possibilities with it is awesome. I go either way on it, but am still planning on selling my GNX3. I just dont use alot of effects. reverb and delay, thats it.