The box it comes in is fairly large; but in addition to the mouse there is a receiver which plugs in via USB or PS2 (via adapter). That receiver also serves as a recharging station so you won’t have to buy many batteries. Speaking of batteries it also comes with 2 AA NiMH batteries suitable for using with the mouse.
About the only downsides to the mouse is its price ($70 in most stores, $50-$60 online) and that there is also an AC adapter which has to be plugged into the receiver so that it can recharge the batteries in the mouse.
After installing all the software included (remember to not install MusicMatch or the other crap) I had to still go in and do some tweaking. By default it turns on mouse acceleration, and has it set to a really slow speed. After that I had to modify the wheel click to do the standard wheel click instead of the funky one it does.
Other than that the mouse is great and I’ve had no problems with it.