I used the MusicBrainz API a while back for a side project that got me sued for some reason (http://tcrn.ch/2rEox3h).

As I recall, it was pleasant to work with and did what I needed it to quite nicely, aside from a feature that my code had depended on being removed — anonymous/unauthenticated search — at which point the project was already basically dead and not worth trying to fix (that was just the last nail in the coffin). In any case, nice to see that it's still active.