This is my second similar Microsoft portable mouse - the first now lives somewhere in Peru!
Buying a second, near identical mouse was a no brainer. I think Bluetrack optical technology is brilliant. Mouse works on any surface (really, really any surface!), so it is great for travelling - sitting up in bed, in an aircaft, on a train, at a formica topped table, etc, without any need for a mat.
Mouse is a little small for my (large) hand, but perfectly useable and very portable - and easy to lose! Battery life seem nearly infinite - a year on and I have not changed a battery. Wireless USB plug is tiny and stays plugged in on my netbook at all times.
The mouse is not a replacement for a decent, weighty, ergonomic, wireless desktop mouse with lots of buttons and sensuous curves, but it is reliable, lightweight, highly portable and it works anywhere. Oh, and it's good value! Can't ask for more.