I've had the 3.5.1 update for a week. Here's my observations:
Call quality is better
Messaging still doesn't work with my Atrix 4G
If I turn off the car with BT Audio on and then turn it back on, radio starts playing for 10 secs or so before BT reconnects
Address voice recognition is slower but more accurate
The "View" button gets moved next to "map mode". All the buttons including "zoom in" and "zoom out" are bigger. This is somewhat annoying because they take too much space, especially in the turn list view.
Route preference can be defaulted to "Fastest Route". So you don't have to hit "Start Route" every time.
Navigation is more stable and seems to be able to restart without locking up the whole system. I used to have a lot of reboots when using Nav. Now it just goes back to the main MFT screen, shows "Navigation Loading" and comes back online in about 20 secs.
MFT still crashes when using Nav and playing mp3 from a USB drive at the same time. It doesn't crash when playing BT audio. I guess it's due to the high system load since playing mp3 requires MFT to decode the mp3 files rather than the phone doing the work.
Lane guidance is added to some freeway exits but not to local streets.
My Climate worked fine on 3.2.2. The new problem I'm having is the fan setting doesn't "remember". Every time I turn the Climate on, it defaults to "Auto"