I've heard a lot of good things about the 640, only maybe questionable things are battery life is decent, and call quality while in vehicles some people have had troubles with additional noise (background).
But it does support multiple devices, so that might be a plus if you were going to use it with other phones or such.
The new 645 I was told doesn't...
I need to do some more research on the jx10.. haven't read up on it yet. I will try though.. but heard a lot of positives for the 640.

I currently have a plantronics explorer 320 bluetooth head set and fine it to be very good quality but want something a little higher end. I also want a bluetooth headset that can be worn without an earhook like the two mentioned here. So out of these two bluetooth headsets which would you guys say is better and why? Would you say one is better then the other or are these about the same level? My phone is a sprint pcs samsung a920 if this matters at all? BTW I love this phone anyone else here have one of these,what do you think of it?