Also see: http://chordbuddy.com/.
If I were to use one, I'd use Chord Buddy because there is an extremely objective learning process: take one chord-helper out at a time until you need none.
Having a source for songs to learn might be cool for some people, but the drawback of having to learn by site rather than touch makes the gTar something I'm not interested in.
Awesome project, though. Godspeed.