Jack Johnson is one of those generic-mainstream-pseudo-rock-alternative-pop shitty guitar players who've hit the scene lately. This is really the kind of music thats targeted at high school kids that want to be all musically pretentious by saying that they listen to "Jack Johnson who's one of those real modern day poets and true guitar genius artists." I frankly hate it, its not talent, its all the same, and there is no skill in both the playing and the 'singing'. Something Awful dude David Thorpe summed him up well in that article he wrote about Jack Johnson and John Mayer, check it out. It might be just a joke, but in some points he really does hit the nail on the head. GIGANTIC PUSSYS!!!!! Not that music can't be expresive, just that we're going a little to far in allowing lyrics like "you're body is a wonderland" to reach the concesis of young people, who may go off thinking this is the right way to think
I think the underlying thing you need to look at before asking a question of whether 'so-and-so' is good, look at what label they're on, look at how popular they've become in a short amount of time, and then you should get an educated opinion. This type of 'music' is the shit that makes kids think they can be rich and famous without knowing what talent is.