I second the rocksmith notion. I DO like rockband more for a fun/party atmosphere, but rocksmith is also pretty fun.
I had NO idea how to play guitar before I started. And I really still have no idea, but I've learned some chords and the riffs to a few songs, so that's cool. I've maybe put..4-6 hours into it? Not a ton. The mini-games are super fun, and the game itself is very encouraging. There are times where I feel like I was completely terrible on a song, and it's not like "YOU FAIL, YOU SUCK" there's cheering, and tons of points, makes you feel good.
Which leads me to my one criticism so far: I feel like it is speeding me along too quickly. I am certainly no guitar prodigy, but because it is so encouraging, it's all "Hey! Great work! Try the next song! And some Chords! And some Palm mutes!" and I'm still wrestling with the "I can't hear you" main riff.
Also, my fingers hurt after playing for a bit, but that makes me feel more hardcore, so take that as a pro or a con.
So that's my 2c.