This game is awesome!
And yes, first of all it is a game, but it has taught me the basics of guitar from absolute beginner to... well, here: I'm no Slash after 100 or so hours but I'm starting to get the hang of it.
A few caveats:
1. A Real Tone cable is needed to play (about 30€/$/£ on Amazon), but I understand a regular audio cable should also work (it should be a little cheaper). Needless to say, you also need an electric guitar/bass
2. DLCs make this game great, and there's a mind-numbing amount of them (with more coming every week), but they also make the game fairly expensive. The best bang for your buck is the buying the original Rocksmith (2009) and the Disc import tool which is a Rocksmith 2014 DLC. (During sales this should be a 30 songs for < 30$ deal).
3. You can game the scoreboard in a few ways, eg. playing more notes than you need to. If you're serious about learning you should check yourself to avoid that.
4. The game has a high skill cap, to put it mildly. Set a reasonable timeline to get good :)
5. In spite of all the "negative" things I've said, this game is the hottest thing on my library and one of my favourite Steam purchases ever! (Also my first positive Steam review actually)
So have fun learning guitar! Or learning new songs if you can play already!

Terrible AI, terrible level design, terrible gameplay and awful dialogues.

Seriously, go play Beach & Clear or Frozen Synapse, this game does not deserve its price tag.

The first playthrough (1 hour? 2 at most) is somewhat challenging, in spite of being ridded with bugs.
You can ace the game without much difficulty on your second try, and after that there is little point playing more.

The concept itself is interesting, but in my opinion the "game dev semi-tycoons" who made it just didn't put enough effort into the game as much as they poured into marketing it (google "game dev tycoon piracy pr stunt" and you'll definitely find what I'm talking about)

My rating is 5/10, wasted opportunity