I currently own two Logitech Anywhere MX, one for office use and one for home use. I previously had microsoft's first explorer bluetrack mouse, so I can give you a comparison of usage.
Both bluetrack and Darkfield work very well in tracking on weird surfaces.. I tend to lie on my bed to watch shows/surf net and used my home wireless mouse for this purpose. Both mice worked reasonably well tracking on my bed's surface, though I find the darkfield tracks slightly better when the lights are off entirely in my room.
The reason why I changed from the microsoft bluetrack mouse is because it dropped a number of times ( my current anywhere mx has also been dropped ) from my bed when I fell asleep and pushed it over accidentally. Unfortunately the bluetrack mouse's middle button and scrollwheel started to fail after a couple of falls ( the anywhere has survived without any problems after quite a few falls. The battery compartment cover pops off when I drop it but it is easy to put it back on and the mouse still works perfectly ) Though I definitely do not endorse dropping it and you might be unlucky and break yours just by dropping it once.
Obviously, dropping the mouse is my own fault and other than the fact that it had problems after a few drops, the microsoft's mouse isn't bad at all.
My Anywhere MX in the office is used on top of a desk calender ( the type with the transparent shiny plastic top covering it ) and it has tracked perfectly well on it. Battery life is decent. My office mouse is used for long periods everyday and the 2 AA batteries needed to be replaced only after 4months.
Overall I would say that the Logitech Anywhere MX is defintely a good buy. The bluetrack isn't bad a mouse either. The logitech however has the hyper-scroll mousewheel function which is extremely useful for scrolling long documents/webpages which I have found to be invaluable for my office work ( usually have documents 30- 200 pages long ). The mouse wheel can also tilt to scroll side to side which is another feature that is very useful.
