Previously Spotify has been available on Windows Phone 7 as a third-party app. This Windows Phone 7 version was actually funded by Microsoft in an attempt to get the music service onto its mobile platform ASAP. Now though, it's officially available on the likes of the Nokia Lumia 920 and 820.
All the usual features are present and correct, including the ability to create playlists, browse those made by friends, and search for new music. And an offline version is available, for listening to tracks while on the tube. (Of course you could always take advantage of the free Wi-Fi.) You can sync on the fly, too, which will mean every song you add to a playlist will appear on your mobile as well as your PC. The usual millions of tracks should be there for your perusal as well.
The app is free, but you'll need a Premium subscription to use it, which costs a tenner a month. If you download the app, though, and log in, you can get a free 48-hour trial of the Premium service. You can also extend that to 30 days by following the instructions once you're logged in.
Early user reviews seem largely positive too. The only downsides seem to be a lack of NFC support, no Last.fm scrobbling, and no Live Tile. But generally people seem pretty chuffed with it.
Give it a whirl, and let me know of any bugs you encounter. There are bound to be a few, as the developers are still working on it. Are there any features that could have been added to help it stand out from the Android and iOS apps? Let me know in the comments, or on Facebook.