The problem with all disposable battery lights that I have ever seen is that they gradually grow dim in relation to the battery voltage. When using rechargeable batteries they are never quite as bright since Nimh or Nicd batteries are 1.2v per cell as opposed to 1.5v for alkaline. Li-ion rechargeable lights maintain the same brightness for the entire run of the battery. The only down side is they give you no warning when the battery is getting low. They just shut off. But I just keep a charger where I park the bike and plug it in every night or 2. I'm running this
Dosun tail light (just came out this year) and really like it's 270 degree spread.