I don't know for sure what caused me to buy this thing - as I don't talk on the phone in the car that much, and when I do my headset works fine. It is a hassle to put it on, and this is my first bluetooth phone, so it's probably a bit of gadget-itis that's infected me.
In any case, I bought the Mr Handsfree Blue Vision for Treo 700w. Not sure why it would be specific to the Treo 700w, as it's just a Bluetooth speakerphone, and as such should work with any Bluetooth-enabled phone. It was pretty easy to setup and "pair" with my Treo. Almost immediately I got complaints from my callers though - as usual for speakerphones used in the car. You know the thing - people who can hear you just fine, but hate being on the speakerphone, so they say something like, "I can barely hear you." Ugh.
It's pretty neat how it automatically connects when you dial a number or when a call comes in, but if people keep complaining - even though I've listened to myself recording my voicemail message and it sounds fine - I am probably going to return it. Speakerphones in the car are one of those things that you want to work, but which in practice always disappoint - nothing new about Mr. Handsfree.