I despise pop/radio, apart from an occasional guilty listen to Lady Gaga. I've found tons of fairly obscure stuff in a range of genres- I've built playlists for brass band, western swing, dixieland jazz, stride piano, period instrument baroque...I haven't tried polka yet, but now that I think of it..
It looks like their catalog is much wider than it is deep though. For example, search "Miles Davis" and you get a good handful of his most well known albums, and a couple of lesser knowns but nowhere near his entire catalog. I think I'm going to try subscribing just so I can delete all the mp3s in memory which are taking up precious space which could be used for...stuff.