I purchased my Microsoft Sidewinder mouse from Newegg some time ago (could be almost 3 years). I have been very pleased with this mouse. Call me old-fashioned, but I like things that are heavy and feel solid. None of this 'Oh, it's so light and small' hype for me. The Sidewinder is a heavy mouse - but only if you want it to be so. It has this nifty feature that allows you to add and remove weights as you see fit. It can hold three of these weights, which come in ten and five gram sizes. Personally, I have three ten gram weights in my mouse.
Another cool feature is the replaceable feet. Almost all mice these days have some kind of Teflon (or equally slick material) that they use as low-friction points for ease of use. The Sidewinder has these feet as well, but they are removable and it comes with three different kinds that provide different degrees of friction. I assume this is so it can be used on many different types of surfaces without inconsistencies. This brings me to another point - tracking. Mice tracking has vastly improved since the PS/2 ball mouse era. First we had optical mice which worked fairly well, but were horrible when it came to uneven surfaces such as jeans. And then the debut of the laser mouse (which the sidewinder is. I wouldn't be talking about it otherwise) which introduced much higher speed at greater accuracy. The Sidewinder can support up to 2000dpi, which can take my pointer all the way across my 22-inch screen with about an inch and a half of wrist movement. The only problem with this is when I load a videogame and begin sniping. Oops, I forgot to turn down the sensitivity and now I have to exit the game and fix that. No longer! The Sidewinder has on-the-fly dpi switching which works like a dream (even without the in-box software installed) and the dpi you have chosen even shows up on an LCD screen built right into the side of the mouse! Talk about awesomeness.
The sidewinder also has the ability to record macros. If you don't know what a macro is, I highly encourage you to read the Wikipedia article about them. The mouse has a flat black button on the side near where your thumb would rest that controls the macro recording feature. Just click it, perform a series or actions, click it again, and congratulations, you have created your first macro. Near the macro button are two small silver buttons which by default operate the forward and back buttons in your browser (but can be reassigned to operate any number of different features through the in-box software).
There was only one thing I did not like about this mouse. Underneath, the laser sensor has a nice metal ring around it which I assume is for protection as much as looks. This ring likes to scoop up tiny dust bunnies. The ramifications of this are that the mouse pointer will randomly spaz out until you flip the mouse over and blow on the laser sensor/emitter. This doesn't happen often, and would probably happen less if I were to dust regularly, but it does bug me a tad.
Some additional things of note:
The Sidewinder also has good ergonomics. It's almost as if one of those ‘handshake' mice was laid on its side. Although it may look ungainly, it fits the palm of my hand like it was meant to be there.
It has a nice, solid aluminum scroll wheel.
There are red LED's under the mouse, which makes it look like it could kill something (a plus, especially for the LAN parties)