Lightness and longevity aside, we all should be using rechargeable batteries with the mouse. Batteries are not recyclable and end up in landfills where they leach toxins into the environment. I keep couple of AA batteries charged up and within easy reach; takes but a few seconds to pop new batteries in. I haven't timed the longevity but the rechargeables seem to last couple of months.
Yeah, I've been using a lot more batteries myself lately thanks to this mouse, my keyboard, and even wireless headphones. I'm actually purchasing a set of rechargeables this week.
(Fortunately for the Bay Area, there are a lot of places that will accept old / used batteries for drop off and proper disposal: Walgreens, any hardware store, etc)