Sonic adventure 1 indeed had a great soundtrack(beside the crush 40 stuff), mainly because it still sounded a lot like you were hearing classic style music with real instruments instead of 16-bit sounds.
Sonic adventure 2 however just was a bit too much for my taste. If you grew up with the genesis games you would never think that sonic music would sound like that in 2001.
A little bit of guitar here and there is fine. I love how they did it in sonic adventure 1, from emerald coast to final egg(which is btw one of my favourite sonic songs ever). Even though there were some great songs in sonic adventure 2, most songs sounded bland and uninspired, kind of samey with a little bit of variation, which is also my gripe with crush 40 music. I don't think the genre of (80's or whatever it's called) rock suits with the sonic franchise
I repeat though, I still think there were some great tracks here and there in that game, just the direction wasn't exactly what I liked.
But this is all in the past anyway, what do you think of the new direction of sonic music(being sonic unleashed/colors)?