I've had
the GR-30 for about 6 months now and I think it's pretty amazing. Never had any real problems with tracking delay (and I play some pretty fast stuff), except on the low E string. So, usually if I need to play a lot of low fast stuff (like a bass line), I'll just shift the pitch of the patch down a fourth and use the 5th string as the 6th, the 4th as the 5th, etc.
Anyway, the sounds are great, you just have to play very cleanly (it'll translate half-picked or accidentally hit notes as glitches), which you should do anyway. You also have to consider the instrument you're playing; for example, if you're using a piano patch, don't strum the chords, pluck them. Don't bend notes on an organ patch. Don't play a low E on a trumpet patch, etc, etc. I think the best sounding patches are the ones that aren't naturally occuring (the synths) or that are similar to the guitar (basses, sitars, classical guitars, etc). But I use the bass and piano patches on just about all of my recordings, and I use the guitar patches live to switch to alternate tunings without having to retune the guitar (you can shift the pitch of each string with the synth).
So, if you have the money, and you don't feel like dealing with a keyboard player, I'd highly recommend buying one. Your audience will be impressed, and you'll have a much wider tonal range to spur on your creativity.
Ryan