I bought Rocksmith a few weeks ago. It's been a really good experience for me. A friend of mine was given a Fender Squire Strat as a tip, so I bought that off him for $50 new. I think I got the game for $30 or so, with the Realtone cable.
I also picked up some DR Neon multicolor strings, which are the same colors as the game, so that's been extremely helpful!
I love learning some guitar, and I just find it great that Ubi at least is still backing a true learning tool disguised as a game. I know it's been out for a while, but I really wish more companies would provide fun learning tools like this game! The arcade mini-games are a nice touch.
The main game plays much like Guitar Hero / Rock Band, but without that icky "wtf did I just spend my time doing?" feeling afterward. I'm constantly disappointed in Harmonix's decision not to include real guitars into their game, so I really hope Rocksmith actually supports some kind of drum kit in their next game. Ironically, the drums in RB ARE already like the real thing, so leaving out real guitar as an option has been kind of silly, to me.
Oh, and so far I've only bought one song pack (Pixies) and I could see buying a few more of those.