Because it's wireless, the Mighty Mouse needs power, which comes by way of two AA batteries. Apple specifies that two batteries will give you five months of power with average use. You can run on one battery, but you'll naturally have to replace it in half the time. In addition to providing the juice, we like that the batteries give the Mighty Mouse a little more heft than the original, which felt a bit insubstantial. The difference is only a matter of an ounce; the new model weighs 4.5 ounces with two batteries (to the original's 3.5), but in a side-by-side comparison we were able to tell, and we like the feel of the wireless Mighty Mouse better.
As with the previous Mighty Mouse, Apple covers this model with an industry-standard one-year warranty. Free phone support is available for the first 90 days after purchase, and Apple's support Web site offers troubleshooting advice, FAQs, and a searchable users' forum.