I have been tinkering a bit - I tried the Treo 650 (I have been carrying the Verizon PPC 6600) because of the somewhat easier phone usability. In the process of doing that I saw that the Parrot CK 3100 was advertised on the Treocentral accessories page. So I ordered one.
In Birmingham AL I could not find one single installer willing to do it so withe help of a "chap" in the UK at JLS Vehicle Security I was able to install the system in my car "professionally". Not only did it look great! but it works FANTASTIC. Sound through the car audio. mutes the radio or CD and BEST of all does voice dialing over the bluetooth. Not only that when I recieve a call I have a nice big caller ID display on the dashboard just out of my line of sight for driving.
I love it. I'm not sure but I suspect it will work well with the 6601 and 6600 but I have't tried mine yet since I've been carrying the Treo.
PS I do travel out tof country some so that is another reason for using the unlocked PalmOne version of the Treo 650.

If they just had the Treo form factor with the Windows Mobile OS I think I would have reached the ultimate THEN!!