Ditto the Cygolite Hotshot for bang for the buck. Depending on the mode it'll last several hours per charge to a claimed hundreds of hours per charge. I tend to use the most economical triple strobe mode in daylight on every ride, slower pulse modes at night, and only need to recharge once a week or so.
Only problem with the Hotshots is they're almost too bright for my casual nighttime group rides -- it's very distracting to fellow cyclists. So two of my three bikes have lower output Planet Bike or similar combo light/reflectors (using AA or AAA batteries), or I'll set the Hotshot to steady dim. Unfortunately Cygolite Hotshot's all peak at maximum output on every strobe mode, so the only way to dim them is to use the steady mode.
Also, the Hotshots and similar lights have mediocre side visibility. So if that's important plan on using more reflectors or additional lights for side visibility. I'm leaning toward adding colorful LED wheel lights to my hybrids for errands and casual group rides. I've seen several folks around town using these and they really enhance visibility without being excessively bright.