So I've played about 35 hours of Rocksmith according to my Steam. In my opinion, it is not a great learning tool the way it is advertised. Can you learn some things from it? Yes, but the way it is advertised is completely false, I highly doubt its the "fastest" way to learn the guitar. That being said, the game is fun to play, although I have had a hell of a time with latency issues and so have a ton of others, so be careful of that. Depending on what you are into, you may or may not like the amount of songs available. One thing I believe makes it a worthwhile buy if you find it on sale, the amount of real pedals and amps that you can simulate is amazing, I spent many hours just playing backing tracks in the background and using different combos.
The arcade "exercises" don't seem to be more useful than doing actual warm-up exercises. Learning songs can be a pain, I found myself looking up tabs and learning the song THEN playing along to Rocksmith, which should tell you right there the system is flawed. If the game had tablature for the songs it would be a step better.
TLDR: Not a great learning tool though the game is fun. I still believe the "fastest" way to learn guitar is old-fashioned hardcore practice, despite Rocksmith's claim.