I own one. It has some good points and some bad points. If you already have a Cygolite 850 I'm not sure it would be worth changing.

Good
- Neutral white led is easier on my eyes than the usual cheap blue led, and easier on oncoming traffics eyes in my experience (I've also tried having someone ride towards me with it)
- Perfect battery - you charge it in light, but can pop off the back and remove it and replace it with another battery

Bad
- Their "dual distance beam system" uses a design that puts an additional beam of light in the nearfield by the bike. In theory it's cool but it's overly bright and in my experience it's to bright and makes it harder to see down the road.
- The runtime is very short on high, either 1:00 or 1:20 depending on the battery used. All lights of this size and shape have short runtimes, though usually they're 1:30.
- The mount is ok, it generally stays in place for me, but it's not great. I don't have any huge complaints (their previous light's mount was terrible) but I prefer a more solid mount.

If you already have a light that works I don't know that it would be worth changing.