I had one of these that came with my iMac, and I didn't like it at all so I tossed it in with my Mini as an incentive. Today, however, in my quest to make the single lightest work travel bag in the universe (outside of, um, people who don't travel with a laptop), I picked up another one to replace the MX Revolution that I usually bring.
No dongle/pure bluetooth is a huge plus. The new Logitech dongles (like the ones that come on the PMX) are tiny and much better than the old ones, but since the 13" MBP only has two USB ports, I didn't want to give one up to use for the mouse.
So far it still feels too thin, but after an hour or so with it I'm really getting the hang of the iPhone/Touch-like finger controls. Having pinch/pull and all sorts of scrolling and gesture options right on top of it is pretty awesome, and I think if I can get my hands used to the size w/o giving myself carpal tunnel, I may end up picking one up for my iMac as well.
You think it might be good enough to replace the almighty MX Revolution???
I messed with the magic mouse in Best Buy for a bit, and I found it entirely too thin.