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.
My Treo 750 works great on the 660 via bluetooth. But my entire CONTACT LIST From the Treo is not carrying over. My missed call list populates as does the recent calls.
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.