RockSmith is a fun tool to practice with but I'd be very hesitant to recommend someone use it as a method to actually learn the guitar.

The biggest problem is that by learning differently, you create a real barrier between yourself and other musicians. Most guitarists learn to play in a somewhat similar manner. You learn to follow along with music and play by the tab or sheet music. The feedback you get is solely from your own ear. You learn how to stay on tempo and key from auditory cues, not visual ones. RockSmith changes all this.

RockSmith is useful to practice really difficult passages, but it becomes a major crutch at all other times and I really can't imagine someone become a well rounded guitarist while depending on it.