gdgt

Pros:
Sensor allows for use on glass, fabric, or anything, and is very responsive.
Mouse is reasonably ergonomic and of substantial weight that feels right
Friction free scrolling is enabled/ disabled by clicking the scroll wheel, and allows
for fast scrolling on long pages
Special button for a custom Logitech feature that mimics the Mac Expose
Can run on a single battery in a pinch
On/off switch is easy to move and protects the optical sensor
Unifying adapter means one USB port is taken, but you can connect mouse,
keyboard, numberpad etc. to that single position
USB adapter is low profile - I never take it out of my laptop
Software tracks battery life very precisely, so you can check before a presentation
or trip rather than just replacing batteries to be safe.
Forward and back buttons are easy to access, but not in the way.
Middle of the road size - very portable, but not overly tiny.

Cons:
No middle click, but can reassign Expose button if you want it back
Friction free scroll is so friction free you can scroll by mistake as you lift your
finger off the scroll wheel or move the mouse around.
Requires a dongle - I miss my bluetooth dongle free device
Mouse shape is generally ambidextrous, EXCEPT the forward/ back buttons
Price - twice as much as the last wireless mouse I bought

Overall, the mouse is quite expensive - over $50 - but if you can forgive the lack of middle click and tolerate a dongle, I can't think of anything else I would ask for in a mouse.