I just spent 28 days or so with the HS820.
Here's a quick recap:

Pros:
- sound quality was generally excellent. Audio was a little low, but no one ever complained about hearing me
- Ergonomics are good, very comfortable

Cons:
- battery life was less than expected. Quite often I would try to use it and it would just beep at me, I assume it didnt have enough juice to work.
- lost its pairing with the Treo once
- clunky recharger connection

Overall I liked it, it was better than any wired headset I've used in the past.
But the power issue was a big one for me. I ended up returning it. The point of having a headset is to be available when you need to talk in the car. Does anyone make a headset with a cig lighter cradle or similar so that the headset is always charged when you need it?

I'm thinking about the Treo wireless headset so at least I can plug in the treo's car charger if I need to juice it up.

Is the HS850 any better in terms of battery life?

Also, there is a new headset coming out, the HT820, which has built in stereo headphones. That looks kind of interesting for non-car use, since the Treo is not the world's most confortable phone.