I got one a week back and I'm very impressed with it. As far as the reviews I read, MK320 and MK260 have excellent battery life as well as range. I can't comment on battery life since it's only a week but the range is excellent within my room and doesn't miss any key stroke or mouse movements. Also, the mouse has an auto standby feature when it's idle for a few mins to save battery, so I'm assuming the same would be present on the keyboard as well.
Only thing to keep in mind is the mouse is a little smaller than regular, if you're ok with this, go ahead it's an excellent buy.