I've got a T610 with a sony bluetooth handsfree kit, I had the handfree fitted for me but watching the guy fit it, it wasn't difficult. The speaker on the kit is not great but it acceptable.
I've had the odd problem with the phone crashing, and a mate has had to send two back with faults.
might be worth checking out the technical advise section at expansys
to see the problems people have had with both phones