Greg,
I should think they would work best with other Sony Ericsson products, like the P800. If you are thinking of getting a P800, bear in mind that there is a new model out soon, the P810. So you should see a price reduction or special offers to compensate.
The HBH-30 has a replacement lined up as well; the HBH-200. The basic design is the same, but the new model has a caller ID display as well. Go
here for a piccy.
The price for the HBH-30 is pretty good, check
this out.
Though the P800's Bluetooth stack is functional, the newly released T610's Bluetooth implementation is more robust. It doesnt have any of the P800's PDA features, but that may be a pro or a con for you.
Also, dont forget that although you get freedom from wires, you will need to access the Bluetooth receiver to charge it up.
Note, if you decide against bluetooth, the P800's handsfree socket is not compatible with Autocom. Instead, you will need to get an adaptor block to fit into the bottom of the phone.
Mike