Here's how the game mostly works. The game requires a special cable that allows you to plug your guitar into your computer. So just downloading the game won't allow you to play it unless you have that special interface cable. Once you plug in and tweak your options, the basic campaign will give you the simple "Rocksmith" experience. When you first start, all of the songs are not yet "leveled up," so you only need to play just a few notes of the melody, and probably no chords at all. But as you play each song again and again, the game figures out which sections of the song you're getting good at, and increases the difficulty, add more of the melody's original notes. This goes on continually, for every song you play, making the game increasingly more challenging the more you play it. After you have played each song and "leveled them up" to maximum, you'll basically be playing the song as it was recorded.
As a "guitar instruction" method, I really think this has potential. The early part of the game is simple enough that really ANYONE could play it, and the more you play, the more the game challenges you as a guitar player. Each song as a few different arrangements, some focusing on lead, some on rhythm and chords. The game is a harsh judge, so make sure you hit EVERY note, not just the ones that are easy to play!
As some who already plays guitar, I found the game was most rewarding when I was selecting individual songs to play from the game's catalogue, without bothering with the campaign at all. I sort of knew how to play Outshined already, but learning how to REALLY play it without cheating was a treat. I pick a song I like, and practice all the parts of it individually until I can play them all, then I try to put them all together by playing the complete song on the maximum level. Sometimes I can pull it off, depends on the song.
So if you're already a guitar player, or you are really interested in learning to play, this game is definitely for you! Tons of new songs to play as downloadable content. But if you're a "gamer" who's not really much of a musician, even if you really liked Guitar Hero, I'd say this game is probably not your cup of tea. It's more of a "musician's thing" than a game.
The game also features support for basses, and an entire campaign just for bass players. FINALLY, some respect!