Just got my new mouse in the mail: a Logitech MX620 cordless laser mouse (from Amazon). So far (about 20 seconds) I love it:

• It's very sensitive. On a "4D Steel" gaming mousepad the slightest twitch sends the pointer flying. I've had comments about how high I set the sensitivity of my mice and this makes me feel out of control. Awesome.
• It works out of the box. No charging (it came with separate alkaline batteries), no messing around with drivers (I'm on Ubuntu 8.04) and no cables -- just a little USB dongle into the back of my workstation. Even the few buttons (forward, back and search) work without configuration in Ubuntu.
• It is comfortable. For a righty, only -- it's molded to be used by the right-hand only, but my hand fits around it more comfortably than the standard Dell or Microsoft hand-blob.
• The scroll-wheel "click" can be turned off. This is the best thing, I think -- a little switch on the bottom of the mouse turns off the scroll-click feeling under the wheel, allowing it to freewheel like a BMX in mid-air.

It's probably all a matter of personal taste, but for me this is an ideal mouse. The more expensive models may have more buttons, or software-control of the scroll wheel, but I don't think I'd ever turn it off (even if I could find software for GNU/Linux).I'll write up a more thorough review once I've been using it for a while.