I have several years of guitar background and I played that game extensively. Today I'm convinced that this is a really fun game (even for absolute beginners, I tested it on friends) but it is NOT a good tool to learn to play guitar seriously.
It lacks one of the most important feature of music : rhythm information. So I'm sure that any absolute beginner that would learn only with this game will have issues with the rhythm notions. Moreover, the learning tools (slowing the song, simplification of the tabs...) are not really well done and won't really help a new player, and might even frustrate him.
The lessons videos are quite good though.
But a real beginner might have another opinion on that one.
So, this game is fun and might be a good addition to a more "standard" way of learning. My best advice would be to work on the songs with a real teacher, maybe using the game as a support tool.
If you don't want to learn with a teacher, at least get good tabs (with rhythm, not text tabs) of the songs and a good tab editing tool (Tux Guitar if you want free & open source, Guitar Pro if you have some money to spend), because they'll be much more useful than the game, but less fun :).
Based on my personal experience, I think that learning with a teacher is the most efficient way to learn (at least at the beginning). A few basic lessons would save you a lot of time.