I think its a great game. There are not many songs that I want to learn on it. But I still load up the game constantly just to use the tuning menu so I can play freely and basically use it as a cheap alternative to an amp.
The best way to learn is to want to play songs you really like, now either way I would get rocksmith, if it has songs you really want to play, then do those and mini games and such. If not, do some of the songs and try to learn songs on your own. I would recommended not changing the tune of the guitar too often, if its in Standard, just do the corrections to make it stay there.
Most of the music I play is in Drop D, which is a bit easier than standard tunings. So if that fits some of the music you like, I would look up that.
Also learning to read tabs are important too, then you can actually look up songs already tabbed and then just try to play them. I can also explain if need be.
But either way, I recommend the game lol