Very cool as a V1, with a user experience that's loads better than existing alternatives. Couple feature suggestions (that are probably already on your roadmap):

-User and/or editor reviews are a must. Tab quality tends to range from completely wrong to almost right, and without any kind of community input it can be difficult for newbies to tell the difference.

-I don't know how feasible it is, but a web based version of PowerTab (or comparable solutions) would be great. Old-school tabs are nice, but when you can listen to a MIDI-fied version of a portion of a tab, it helps a lot.