I use thesixtyone, earbits, grooveshark, last.fm in the browser and I have no trouble at all. The thing with music apps is that I just choose a playlist/station, hit play and forget. The UI doesn't even matter beyond that.

For a lot of people with that use-case the UI might not matter, but it is not the only use case for a streaming service. From http://www.csc.kth.se/~gkreitz/spotify-p2p10/ one can see that almost 40% of tracks played are chosen actively as opposed to just continuing on to the next track.

The Windows version seems to wish it was iTunes.

Other than that, its quite responsive.