I can see where they're going with this, but right now it's really only halfway towards something that would make me shift away from my main music player (Winamp, fyi). So, problems I've found so far:
1) The main interface is like a really crappy version of iTunes. I hate iTunes. It's difficult to browse, and it doesn't even appear to include different ways to browse the library than by list, something which even iTunes manages (coverflow, by genre, putting lines between albums even, etc.).
2) 'Linking' tracks. In theory this is a good idea, but it seems to struggle with very minor spelling issues. It will repeatedly fail to link one album track from an album it recognises despite being spelled in exactly the same way as the one they've got listed on their database, and I have no idea why.
3) Compilations. I can't figure out how to make songs from compilations appear together - it's either sort by artist or album. MP3 tags have a useful field called 'Album Artist', which can be filled in with 'Various Artists' or just 'The National' or whatever. So, on Winamp, I have it organised so I browse through Album Artist, pick the album I want to listen to and then the specific track if needed, but the actual artist listed on the track will still be the artist who actually plays it. iTunes prefers to just group things by remembering certain things are compilations. Either way, it works (though I prefer the former because it's a universal standard, even iTunes recognises it, but iPods don't which still pisses me off). Spotify doesn't appear to recognise either of these methods. Am I doing something wrong?
4) Is there a way to make it so people can't see your playlists? Sometimes I'll make playlists that I don't want people to see. I don't like everything about me being plastered up on the web, nor do I like those playlists I make as preparation for mixtapes to give to lovely ladies being up there too for all and sundry to gawp at.
5) They've already got a linkup with last.fm, so they should adjust the 'Top Artists' and 'Top Tracks' on your profile to match your last.fm stats. Shouldn't be too hard to do.
None of these are massive issues, really, and I do like what they've done so far - the link with Facebook is ingenious, frankly. Just hope they sort these kinks out. I'll have to switch between two different music players for the time being.