I'm trying the oiid app at the moment with one of their free songs and I have to say I'm quite impressed. Sound quality is good enough, although it seems to be lossy rather than lossless, you get a music video in the background of the mixer section of the app and the interface itself is pretty slick and responsive. There isn't as many songs as there are on "Mix The Music" and a lot of the songs are the same ones (albeit with less tracks it seems, usually the drums are mixed down to a single track rather than having separate Kick, Snare etc).
EDIT: Ahhhhhh.....I think a lot of the songs that I wanted are only available in the US when I'm in the UK