I have a G2 (no built-in expression pedal) so I expect the sounds are the same. I bought it primarily for use as a portable practise headphone amp and also sometimes gets used as a reverb/echo unit in my pedal chain so I am perhaps not the most critical of users...
The G2 effects are generally pretty good as are the distortions - and the amp simulations are OK (I have tried Boss, Behringer and Line6 and I would describe them all as 'OK') and in the context of the price, quite impressive. The basic sound quality is high - the Zoom uses good quality AD-DA converters.
One minor dissapointment to me was the reverb is not as lush as some - but it is still quite reasonable.
The G2 is quite well made, but the lack of a decent display of course means the usual head scratching trying to remember what each 2 character 7-segment acronym stands for.
You can check out loads of reviews here
http://www.harmony-central.com/Effec...M/G2_1u-1.html
and
http://www.harmony-central.com/Effec...ZOOM/G2-1.html
Hope that helps!