You don't have to wonder if the lights you strap to the hood of your car have enough charge in their batteries to stay lit for the duration of your drive, do you? Oh yeah, they're always working, getting current from the car's engine.

I'd say to really look into a dynohub setup. If you ride often at twilight or in the dark, and as long as you're going faster than maybe 5 mph (slower than that and they'll strobe), they're worth every penny (and sometimes cheaper than the expensive rechargeable setups, even after including a new wheel). They also help add some visibility during the day.

If I added together all the lights and batteries I've bought over the years, the total would've been about the same as outfitting two bikes with dynohubs and lights. Considering that I have three bikes and one already has a dynohub, I wouldn't need any battery-powered lights at all.