Some one recommeded it in a post a couple of months back. I bought one to replace my battered 7 button razor. Very nice. I map the extra buttons to my uoa macro hotkeys (alt+ctrl+shift 1 to 0).
This allows me to easily use unweildy keystrokes for macros, and frees up more keys for other macros.
It's a rather small mouse tho, and takes some getting used to. The placement of some of the buttons are a bit awkward to use, like mouse button 4 and 5 that are to the left of the left mouse button (requiring moving your finger off the left mouse button), and the last 3 buttons on the numpad (too far back).
However, that still leaves me with 9 usable buttons.
It comes with training pads that you can use to get used to the feel of the numpad.
All in all, I am very happy with it.