I love spotify, although their advertising seems to be becoming a little more pervasive in recent weeks, however without a client for Windows Mobile (and the associated subs) I can't justify coughing up.
However, they need to ensure that it supports 'streaming' or 'caching' on the move, otherwise I may as well just continue to copy my MP3's to my phone as and when I want to listen to them.
And for those complaining that you loose access to the music when you cancel your subscription - well, yeah, but then you just sign up with someone else. I don't mind paying £10 a month for someone else to rip albums, setup the album art, make sure the ID3 (or equivalent) tags are in place and then have a decent network connection to serve them up.
One final point though, they need to expand their range of "classic" artists. No Flloyd, No Oasis, No Zepp. Or rather, these artists need to see that music rental seems to be the model of the future and they should embrace these new ways of working, rather than ignoring it, then taking legal action against it, then embracing it 5 years after Napster and P2P appeared.