one word...
awesome.
more words...
even the lead part will regularly be different to what you play in single player, as often times in single player the part you play incorporates rhythm and/or bass at certain times.
playing bass feels very different due to the different types of patterns and generally speaking, a lot less chords. the bass part on YYZ is nuts.
playing rhythm doesn't feel as different as bass does, but it still feels very different. it's weird to be accompanying solos!
your multiplyer shoots up very quickly, which balances out the fact then when either of you make a mistake it resets it quite nicely. the 'shakey' thing that happens when you break a combo makes a lot more sense in co op because it makes it clear who was the one who screwed up.
also, the sound for a bad note on bass is actually really pretty cool. i catch myself strumming at the end of songs just to hear it and freestyle a bit, and have caused more than one fail at 99% as a result.
you can't totally carry someone. if you're playing 100% perfect the other person can still screw up badly enough to game over the song. it feels like the balance there is just where it should be.
it's a great addition that plays really really well. just a shame they couldn't get hotel california and it's two lead guitar solos in there. THAT would have been the ultimate (and i don't even like the song that much).