FYI Jammit has a PC Beta version available now. I just stumbled on this through Line 6 on Twitter. I downloaded the mac version and it's pretty awesome. The interface is nice enough but I haven't found a good way to loop a full section--dragging the loop region is pretty painful--but it might be an RTFM situation
I have been using GuitarPro for quite awhile and it keeps getting better but the use of actual tracks in Jammit is a really nice. With GuitarPro you can get free downloads of just about any song usually with all instrument parts but, if Jammit has a version, I think it's worth the $ to get it. Both have computer and portable versions so you can use either wherever you are.