It worked for me on ubuntu as a free user, even though the site said it wouldn't. I did experience issues with the ads getting stuck visible over the interface, but the audio worked fine.

Yes, it works for free users now - at least it worked for me (I have since gotten an account.) I was using the Windows client via Wine, which worked surprisingly well until an update killed it a few months ago. During the same update, however, the Linux Preview began working for free users. I'd try it again if I were you.

I also use it, and confirm it uses Qt and QtWebkit. Other than some finicky setup to get it to play local MP3s on Arch, it works wonderfully.

Also, for Gnome-using friends, I created a python wrapper for Spotify to map Gnome's media key events to the type that Spotify is expecting, so that I can use my keyboard to control the music. https://github.com/jreese/spotify-gnome