I give it a 7/10, for being the first Guitar simulator (or whatever you want to call it) on the DS. It could have been a lot better though, if they put a bit more time into it. I agree with Chupperson, it's not a game, but an instrument. You can't say, "this game sucks" cause it's not really a game. Don't expect to play it as a game. Expect to play it if you don't feel like learning/playing your guitar.
Edited because.....well, read above. It is fun, and I play it for enjoyment. My bad.