Saw a post about this on TorrentFreak last night and was very kindly sent an invite for the service by Ernesto. 
Spotify‘s tagline is ‘Everyone loves music’, the interface is a media player with an iTunes feel to it so nice and familiar and easy to pick up for most people. Built in collaboration with most of the big record labels, it has a vast library of music streamed from the Internet (around 160kbps according to the TorrentFreak article), apart from some of the more indie stuff it seems to do quite well at finding anything your after.
Its not a complete collection of the worlds music by any means but its growing, the only random things I can’t find so far are the Metallica S&M recording from the live set in San Fransisco (I was listening to it in the car this morning so it sprung to mind as something to search for), and anything by 2manyDJ’s.
Music starts playing as soon as you click on it (I’m on almost exclusive use of a 100mbps pipe at the moment so it might not be quite as fast on a normal connection!) and its easy to find more tracks by an artist, find what compilations they appear on etc. Reviews and Biographies are gathered from a range of sources including Wikipedia and All Music.
Of course the main advantage of Spotify is that a) its free! (some unobtrusive adverts) b) it would appear to be the first credible, legal and usable music service to emerge. The guys behind it seem to be taking tips quite happily from the beta community, Last.Fm integration is there and it would seem that more will follow.
Of course, the disadvantages of the system at the moment are no connection with portable devices, according to TorrentFreak this might come in the future. Of course with  better connected media devices including phones (iPhone, G1 etc) and improved data plans we might end up with a situation where we can just stream whatever music we want and don’t bother keeping it ourselves.
There’s a little introduction video here: https://www.spotify.com/en/about/press/concept-video/
(and no I still havn’t had a chance to sort out the template issues on this site!)
This sounds like a really interesting service, do you have any invites to it?
Sorry, you only get invites if you pay the full £10 a month, you can try commenting on the torrentfreak post and seeing if there are any invites left.