I was in the exact same position a year ago when Rocksmith first came out. I had previously tried to pickup guitar, a few lessons, but nothing stuck - all the normal excuses: boring, fingers hurt, not enough time, etc.
With Rocksmith it enabled me to spend the 2-3 hours a day actually on the guitar. Playing entire real songs (even if just a note or two out of a solo) with skill challenges, point scores, and all the other gamification of practicing. It was no longer practicing to learn X or Y but having fun trying to break the 70,000 score to unlock the next item or get the gold medal in bending.
The parts that Rocksmith lacks are:
- Fretting Explanations;
- Theory; and
[Updated: Rocksmith 2014 now has much better fretting explanations and even a mini "arpageio" charts on the first note to show the suggested fretting to easily play picked notes. Keeping this section, but it only applies to Rocksmith 1.]
What I mean by this is some finger placements are not all that intuitive, while this could be a personal choice for single notes I found I was constantly going online to find a video/teacher to explain chords (in-game navigation menu is terrible and confusing).
Further, looking up how the actual artist performs live or another person showing how it's done also helped - you see the musician holding a Bm barre chord and picking notes instead of trying to finger and move fingers vertically around.
[Updated: Rocksmith 2014 now has a "session" or jam mode with AI instruments for dynamic backing tracks. It also shows the key a song is in and the mode (with options to change). While a little light on the explanation it is there and extremely fun to simply jam out with a band.]
Rocksmith has zero theory. Being able to name notes, progressions, modes, keys, and all the technical parts are a big part of understanding guitar and Rocksmith just does not deliver. I didn't think was a big deal at first, but now starting it a year late I get a lot of "Ohhh that's why..." moments which would have led to a lot less frustration early on.
This is also a requirement if you want to move beyond cover songs that you have tabs for. Jamming/improv, composing, being able to follow along with another person, and so on.
[Updated: Rocksmith 2014 now has "accent" notes/chords where you strike that note with more force, similar to saying "ONE two three FOUR ONE two three FOUR" where the capitalized beats are louder. It doesn't help with up/down strumming, but it is something.]
I personally had a very hard time with learning rhythm songs, as in most beginner songs - I had to spend many many hours outside of Rocksmith learning all the common what I call "down up" strumming patterns. I knew what I was doing in game was not sounding right and Rocksmith gives no indicator if it is an up or down strum.
Another factor is counting time/measures. When I started I was relying on the visual markers and not the beat or tempo, this is especially hard at the very very beginning in that you don't play all the notes and it would sometimes throw off my timing that I didn't realize I had. When learning from a teacher/video for "strum" songs they would often break it down into counting: "1 and 2 and, 1 and 2, and" for the different timings like 4/4 or 6/8. At first I had to actually say it out loud to force my hand, then moved on to tapping my foot - I would have not got that using purely Rocksmith.
As Julien N mentions, all the above can be solved with a teacher. Since I'm just in it for fun as a side hobby for campfires or rainy days I don't mind struggling along for 3-5 years using just books/youtube/dvds but a teacher would be able to fix the incorrect posture/finger placement, give a clear course on theory and help identify and learn strumming along with all the other benefits of having live feedback.