I downloaded it onto the phone yesterday and gave it a whirl. So far I like it an awful lot. It's not perfect - it doesn't really do albums, so you have to right click on albums you want and say "create a playlist" and then tell the phone to "make available offline". But I could create the playlist on the PC and then just go through and select each one on the phone, which didn't take long at all.
It did take a while to make the tracks available offline, although I'm not sure if that's because of the speed of download or because the phone/memory card is slow. It certainly wasn't anywhere near maxing out the broadband or WiFi speed.
The interface is also annoying on two minor counts:
No text prediction (which isn't as annoying as it might be, as most band names aren't real words anyway)
The "hang up" button on my phone returns me to the main screen for most apps, leaving the app running in the background. Spotify actually quits. Which isn't the end of the world (pressing the menu button twice has the same effect) but it's not ideal.
Oh, and their selection of music isn't universal. There's a _lot_ of music there, but there's a fair chunk that's missing too. I'll have to supplement with some albums from my MP3 collection.
But in the meantime I have a big wodge of music on my phone, and more than I have time to listen to waiting next time I feel like something new.