Rock Band and Guitar Hero doesn't actually teach you anything. Many people, particularly musicians, take the stance that the time you spent getting good at a rhythm game could have been better spent learning how to play a real instrument.
Well, now we have the best of both worlds with a game that teaches you how to play the guitar. Rocksmith comes with a USB adapter that will work with any regular guitar, and provides an environment full of amps and effects that function to let you enjoy yourself while you slowly learn how to play your guitar over time.
For people that have never touched a guitar before, the game starts about as basic as you can get. The first thing that Rocksmith will teach you the first time you boot it up is the proper way to hold your guitar, and then moves into how to pluck and tune the strings. For people who have decided to upgrade from a more traditional rhythm game, the developers of Rocksmith have included the highway of notes that scroll by the screen at a rapid pace, although this time they are actual notes.
Most people usually give up on learning Guitar because they are just playing boring songs they don't really recognize. Rocksmith has intelligently managed to completely dodge this problem by very quickly moving from basic lessons on how to make noises come out of the guitar into very basic riffs from popular songs. This keeps new students engaged while at the same time trying to give more seasoned guitar veterans a reason to buy the game.
Sadly, it is with this audience that Rocksmith completely falls flat. The game takes forever to ramp up to the higher level lessons and more difficult songs, and there are no options for people who already know the basics of guitar playing to skip any of the basics, which will often lead to them becoming frustrated. Because of this, the only people who should really be considering Rocksmith are those people who want to learn guitar but have never started doing so.
Beyond the basic learning modes, Rocksmith also features a number of videos that show the concepts that are being taught in-game in a very simple manner that make it far more personal and easy to understand. It also features something that it calls the "Guitarcade", which has mini games that often end up feeling like someone took the basic idea behind Typing of the Dead and shoved it into a guitar game.
Rocksmith certainly is not for everyone. It requires a desire to learn that not all people have, and its certainly not enough of a "game" to play for fun, but for the price it makes a fantastic training tool for people that are new to playing guitar, and it's a lot cheaper than a real person.