I use a Garmin Edge 205 and a traditional bike computer (Cateye Astrale). The Garmin only runs about 8-hours per charge, even less if it spends a lot of time searching for satelites or if the weather is excessively cold or hot, so it's good to have a backup.

Also, the Garmin Edge doesn't have a cumulative odometer... I like to keep track of how many miles I'm putting on my bikes so that I stay on top of maintenance.

Edit: I do have a battery "booster" for the Garmin Edge. It's a portable charger made for IPods, and runs on AA batteries. I bought it on e-bay for about $10 and use rechargeable batteries with it.