Hi,
I play Rocksmith more than I probably should (instead of, you know, actual practice...).
Picked it up mostly so I could improve a bit at guitar.
Had it for about two years and nailed most of the bass versions. I think it's a good tool, especially for "effortless" pratice. You don't have to set up any gear, don't have to get out tabs or books, nothing. Just plug in and go. I think that can be really valuable to many people, given that we all know how much the small inconveniences can keep us from getting stuff done.
There are, however, some things that I believe are worth keeping in mind when using Rocksmith.
First is latency. Even though it's not really that bad with my PC, it is really noticeable to me and leads to me playing a bit before the beat so that I (and the game) can hear myself on the beat. That could potentially mess up your timing, especially for beginners.
Second is learning songs. As intiutive as the interface is (to me at least), it really is useless at helping you remember songs. I don't know whether it is too easy or something but I cannot play by heart a single song which I can get completely correct on Rocksmith. I don't know that phenomenon from regular practice with tabs etc.
The last thing for me is that the game is sometimes to unspecific vis-a-vis the technique you should be using for a song. For Behind Blue Eyes, for example, it tells you to slap some notes, without any hint about Entwistle's real technique.
I am also not 100% convinced of the accuracy of the tabs in the game. I think it is We Are The Champions (might also be something else...) where I have noticed some details missing. That doesn't really take anything from the game but might be annoying if you really love/need a specific song.