Just got the HS-850 a few hours ago. Using it with an XV-6600, and all seems to work well. I have noted difficulty answering calls from a 'boom closed' position. The headset does not reconnect in a timely enough fasion consistently, and calls go to voicemail (after about 4 rings). I have gotten it to connect a few times (from off) but it usually does not. I suppose it would work better if the "No answer transfer to voicemail" was turned off, but I'm not interested in doing that.
There is no problem answering calls at the phone (green button), then opening the boom, the call transfers to the headset within a few seconds.
Aside from that it seems to work great. Pairing went without a hitch, volume could be a bit louder but is not bad, comfort is good.

Also have a Logitech Mobile Pro that works well. Volume is great, connection can be staticy.

I will be getting a Mobile Freedom this weekend. I know a guy who works for Logitech and he gives me this stuff for free!!

XV-6600
BT patch 3100