I use a Motorola HS650 and it works well.  I wouldn't say that I'm
thrilled with the quality, but it gets the job done.  After an initial
period of being a little bit echo-y to my call receipients, it now
works well.  However, as the battery in the headset drains, the quality
deteriorates and becomes more and more crackly.  I will also echo
rsquared's comment of if it isn't line of sight (i.e. in a back or coat
pocket) reception does downhill.  However, if it is  on a countertop
near you, it works quite well.  Hope this helps.

Thanks for the info, John.  I got version 1.04 firmware for the 650 and
installed it holding my breath for a friend, today.  He's on Verizon and
the new firmware seems to have solved a problem he had with HotSync.  The
directions on Treo's upgrade page are fairly complete and DIFFER from
what the ROM upgrade instructions are in the firmware installer software.  
Use the upgrade webpage instructions, not the firmware, and make a dummy
user to upgrade to.

Holding down the power button and pressing RESET in the battery
compartment, then still holding down the power button waiting for that
second Palm screen to come is knowing everything is being deleted in it
is a little scary....(c;

The pictures that were stored in the phone, however, DON'T get deleted,
or they were backed up in the process I followed in the instructions.

1.04 is better firmware.  Sync'd right up the first time with the Palm
Desktop data on his computer.  He's a happy camper as the calendar, phone
and contact lists were trashed by 1.02 and refused to hotsync anymore
until now.

http://www.palm.com/us/support/downloads/treo650updater/verizon.html