I'm a big fan of Spotify and I have a first-generation iPod Touch which does pretty much nothing but connect my vintage hifi to the modern world. It generally works well, especially now I've figured out how to charge the device at the same time. When I'm on the move I use Spotify on my Nexus with the tracks offlined.
However there was one big issue: turning off shuffle on playlists. I was reduced to searching on web to find out. Turns out that you tap the track that is playing to get a new screen and if the Shuffle box is grey then it is active, and white when inactive. That is the only indication. Usability failure - as witnessed by the long list of relieved users who couldn't find that.
I am reminded of a previous iPod challenge - finding the track and album repeat option. In early versions of the iPod touch software this was only visible when you tapped the region at the top of the artwork. This has since been updated to make it visible all the time which is just so much better.
As touch interfaces become pervasive this triumph of design over affordance will have to stop. It's not as if there aren't plenty of simple, elegant graphical solutions that make the options obvious to the user.