If you aren't looking for something like this, that's cool, just ignore my post. But if you're looking for the most reliable bike lights, my dynamo light (energy is produced by the wheel as it spins) is the most reliable light I've used. All battery issues go away, it's not being taken on and off the bike (it bolts onto your bike just like your other components - derailler, handlebars, etc).
In 2008 dynamo lights were kind of pathetic. Here's 2 lights from then -
But here's what you can buy now (2014 Schmidt Edelux II, similar to the cheaper Cyo Premium) -
Note that while they have a really, really great beam pattern for road riding (a shaped beam like a car headlight), that's not saying they're great for mountain biking. The Phillips Saferide I have seems pretty reliable to, also has a shaped beam, but short battery life (only 1.5 hours on high apparently) and to get the latest model (apparently they're discontinuing them as well) you have to order from overseas.
Are you looking for mountain biking or road riding?