I've been playing about with a few things, and here are my findings:
My biggest niggle is that I can't get artwork using my iPhone 4S, even though I'm using a red collar cable. I think others are doing it successfully though.
You can use Spotify using either the MMI cable or by streaming over bluetooth. The track and artist will display on the MMI, but you can't use the MMI interface to scan through your spotify playlists, no matter how you connect.
You can change tracks using Spotify, but it's buggy over the cable, and more solid over bluetooth. The MMI interface also has problems deciding how long a track is when it's coming from Spotify. Changing back to the iPod functionality of the phone and picking a track re-sets the track length and all is well. Switch back to Spotify, and it all buggers back up again.
I've also had a problem where my MMI screen went black for a second, and when it came back, it told me that my iPhone was an incompatible device. I fixed it by rebooting my phone. I think that again it was due to using the iPhone's task management functionality to switch between iPod and iPhone using the MMI cable.
One thing I have been able to do is use the Handbrake application to convert a TV episode into MP4 format, which I then transferred onto my phone using iTunes. I could then watch it on the MMI screen by plugging my phone into the MMI cable, selecting the source as cable, and opening the videos section.
Any questions, lemme know, and I'll try and update the post where necessary.