I've had one for about a month, (replacing a Magic Mouse.)
My observation so far.
1. The feel is great and it is very accurate for my use, (visual effects work.)
2. The Synapse software is a joke. Half the time it hangs at boot for no reason and if you move your pointer over the menu bar icon it just pinwheels. I've tried uninstalling and reinstalling several times to no avail. Seriously skip the synapse software.
3. You can at least use the right side buttons to stage up and down the sensitivity without the software. Although I wish it was the left side as it is too easy to hit them accidentally, and they are harder to find by feel.
4. I tried BTT and magic prefs as I heard they work with it, but I couldn't get anything to happen, (to br fair I didn't spend more than a, minute or two messing with the settings as I don't need either programs other features.)
5. It goes to sleep, WAY to quickly, 15 second of no use and it looses its connection. With the synapse software it would take 20 seconds to regain a connection after pushing a button. When not using Synapse it's more like 4 or 5 seconds.
6. battery life sucks, I only get about a week and a half use out of it, and I don't use it all day long. My Magic Mouse lasted at lease a month and a half. (Have a high end PC graphics machine that I do most of my main work on sitting next to it.)
7. I have a death adder that I love. I wish they made a blutooth 4 version of that, it would be my perfect mouse, (I know there is the Mamba, but it doesn't have bluetooth)
8. They need to update those synapse drivers and release a firmware update to decrease the sleep lag issue.
I do like the mouse, and there are the pros and cons as I see them. I wouldn't mind the battery stuff soo much if it would wake instantly and the synapse software were usable.