See TSB 12-11-01 Posted Today - MFT v3.5.1
for the main discussion on this TSB
I originally had some bluetooth laggyness/skipping when I installed 3.5 but this went away after a master reset. Since installing I've found that my iphone 5 connects faster and more reliably. Bluetooth music playback is greatly improved as well. The system will seek to connect back to it's previous audio source without having to do what one person described fantastically as the "bluetooth dance"
If you don't have nav I'd really suggest downloading it if you are having problems. It has been a great improvement for me. I believe once ios 6.1 comes out and fixes a few issues on their end I'll finally be happy with MFT.