My friend and I have the same problem. The Garmin disconnects the phone 8 out of 10 times. There is clearly Bluetooth incompatibilty and Garmin should not be claiming that they support the Treo. You would think they could come up with a patch. By the way, their newest update software makes the problem worse, rather than better and recently destroyed the software on my Treo 700P, forcing me to get a new phone.
The bluetooth download takes a long time and will often successfuly load your favorites without signaling that it is finished.
Just purchased the 680 and I had no problems sinking between the two devices. My favourite phone numbers were carried into the 680 with no problems and I have not seen any disconnect problems with the phone to date.
I had the same problem with the 680 and the treo 700wx, it would connect and disconnect constantly, enough that I turned off the bluetooth. I did get the treo 800w and it has worked great, still can't text I don't know of any phone that can.