I have a cute gray Targus one, smaller than a regular mouse, but not as mini as the little tiny ones. I love it, but right now it's not working b/c someone spilled Coke on it. Anyway, 2 AAAs would last me a month or two, but I rarely turned the mouse off at night (so I wouldn't have to turn it back on and re-train it each time).
Right now I'm using a corded USB optical two-button mouse (w/ scroll wheel) b/c someone at work had a box of extra ones. Honestly, it works just as well, I just like how the cordless one looks w/ my laptop and it's easier to pack b/c the cord doesn't get all tangled.
As long as the mouse is optical, either one works fine.