I have a Cateye LD-600 rear light, and use recharchables in it, they get charged about once a week (Ok, I swap out with the set that's on the charger whenever I notice that they're getting less bright)

Takes me all of a minute out of my week to swap them, and I have a spare pair of alkalines in my saddle bag just in case I forget. Well, ok... that spair pair of batts usually ends up powering my mp3 player, but I add a new battery when I get home or to work to replace the one I took.