I have subscribed to Spotify for a few years now. (Live in Norway.) I just recently bought a Fostex TH900 and a Fostex HP-P1 amp/dac, and I wanted to get the most out of my headphones, so I'v bought a lot of albums in FLAC. (Most of them in 24bit/96kHz) Today I did a comparison of audio quality. I switched between Spotifys highest setting, 320 Kbps Mp3 and 24/96 FLAC on the same album. I listened to Hello Troll by Helge Lien Trio. Spotify link: http://open.spotify.com/album/7uj2x11XMDZ0IkmooASH7M
(That is a award winning album for great sound.) What shocked me was that I wasn't able to hear any difference at all. At least in the beginning. When I began listening to the faintest details - like in the echo of the percussion and the delivate sounds of base strings, I could make out some small differences. But it might just be my imagiantion.
My conclusion is that I'v wasted a lot og money on high-res files. At least on those albums that are available on Spotify.