Ok had a play with internet radio + bluetooth streaming today.
Pros: Dead easy to set up - already have an iPhone 4S with TuneIn Radio and the Planet Rock app installed. Simply a case of launching the app when you get in the car and then choosing bluetooth from the iDrive media menu. Set off from an area with a 5-bar 3G signal and it started playing straight away. Also, depending on the app you're using the playlist is stored on the phone (given you don't get any info sent to the iDrive) so you can check it when you get home.
No-so-pros: Trip #1 - Travelled from Brighton to Shoreham then up through the South Downs to Horsham (about a 45min). Steady reception until I turned north up the A283 thru the South Downs. Phone reception dropped to 1 bar and the music cut out. It returned after about 20mins when I was nearing Horsham. Trip #2 - When I came to leave Horsham it wouldn't play at all. Turned out the problem was that the audio feature of bluetooth has switched itself off in iDrive for some reason!? Re-enabled it. Travelled along the A264 then down the A23. Signal cut out for about 3-4 mins around Handcross/Bolney plus I got what sounded like clipping around Pyecombe otherwise good. Audio quality - not great if I'm honest. Ok I only have the stock BMW audio but compared to playing MP3s or decent FM reception I'd have to say it wasn't as good. Don't know if HK would help or just exaggerate any 'source quality' issues. Also, the volume on the phone was around 65% and I had to crank the volume in the car way up to hear anything. When I got to where I was going and stuck the car in reverse, the parking sensor bleep was so loud I nearly jumped out my skin!! Chewed thru the battery on the phone - so best have it plugged into a power source (not the audio USB socket) if you're on a long trip. Changing channels - well that would involve fiddling with the phone and the implications that go with that unfortunately so probably better to pick a channel as you set out and stick with it.
Overall not bad but room for improvement, although others may have better luck. All down to expectations in some ways - go back 5/6 years and this setup would have been magical, but today I guess we've come to expect so much more