Rocksmith is awesome. I was on the fence about buying it because of how Rockband 3's "real guitar" feature was so terrible. But once I read up on it, I took the plunge and am thoroughly impressed with Rocksmith. It works with any electric (even elec/acoustic) guitar, and it increases or decreases difficulty on the fly. It will start out with a type of challenge called "single note" (or one note, cant remember) and it lets you sort of get the feel of how the song works. Then once it notices that you're hitting all the notes right, it 'levels up' your guitar part and starts throwing more notes at you. Then more and more until it notices you can hit chords, etc. Then if you seem to level up somewhere and you start missing a lot, it levels down.
I play guitar, been playing for 20 years (but I sound like a newb because I suck) and this game is actually quite helpful.
Rockband 3's "real Fender guitar" is a piece of shit. And RB3's guitar learning section is TERRIBLY designed. It's absolutely atrocious. It doesn't LET you learn. If you miss a note in the practice/learning area, it will STOP cold. Rewind some weird amount, like not even a couple of beats. Just like 2.54 seconds or some crap, and then starts up again - which doesn't let you get ready for the next note, so you miss again... and it stops... rinse and repeat.
Rocksmith doesn't do that. If you miss a note, or even if you don't play a single note the entire song, it still keeps going. It doesn't 'fail' you and stop halfway through the song. If you don't get enough points to 'qualify' for the song, then you'll just have to play through it again. Simple as that. It's really awesome, and it's not even presented as a 'game'. It's presentation is much more like a teaching video.