The mouse itself is a nice (Apple-inspired) concept: a multitouch sensor replacing good ol' mouse buttons and wheel; after enduring several mice with hardware defects, having one (almost) without any mechanical moving parts feels reassuring.
Actual usage is slightly less natural than you'd expect. Scrolling is probably the trickiest activity, since in practice it now depends on friction between your finger and the mouse surface, and you have no mechanical feedback.
Right-clicking is also a bit weird, if you're in the habit of resting your index finger while doing it: the sensor detects "rested index + clicking middle" as a different (void) activity, not as a right-click, so you'll need to actually lift the index as you click with the middle finger, or move all the way and right-click with your index finger.
I believe the two items above are to be blamed for 99% of bad reviews; they break long-standing muscle memory for power users, who are heavy on their scrolling and right-clicking, and give a feel that the mouse will randomly decide what to do rather than respond to actual gestures.
The scrolling speed can be adjusted in the driver properties, reducing awkwardness, but there is no tweak for the right-click problem. And this is where Microsoft really failed: there is no way to easily customize the device.
The driver defines a limited set of gestures that you can turn on or off, but you cannot override their assignment (i.e. what they actually do) nor create new ones. Some of these gestures are quite nice: three-finger swipes will bring up the Windows equivalent of Apple's Expose'; two-finger swipes work like Win+Arrows (probably the best feature in the Windows 7 Aero desktop), thumb swipes perform Back/Forward in a very natural way. Unfortunately there is a long list of other gestures that one would really need (open/close browser tabs, a middle-click replacement, window resizing, etc etc) and there is no way to add them; worse, there is no way to simply remap existing gestures to new commands, which should be dead easy; one can only remap left and right click, which nobody will ever do.