I stream HQ (256 kps or better) from iPhone and Spotify all the time and they sound great. I think ANYTHING sounds better than Sirius - total garbage, IMHO. Some of the DC HD Radio stations really put out great sound as well. So I gleefully called Sirius and told them bye bye 12 months ago.
Total waste of $$$ IMHO. The RNS-510 (VW) will not allow the head unit to control my phone via BT other than volume and play/pause. If plugged into the Media In cable then it's an iPod (although VW's older iPod interface totally blows too). Hopefully the BMW HU will - you wouldn't believe what a PITA it is otherwise.