I had one of those, the batteries lasted a real long time, in fact I eventually threw the mouse out – when the button’s springs wore out. So there’s a down side.
Older wireless mice would drain their batts pretty rapidly. But the new one I have lasts at 6-9 months between battery switching. It’s one of the Logitech Invisible Optic series.
'Couse, if you use the mouse to play mouse-intensive games, like for me, Diablo II, then forget it – those will suck the battery charge out in a matter of weeks.