I like the power and flexibility CR123 battery flashlights offer. But I've never been thrilled with non-rechargeable batteries. The bottom line is I can never store enough. Well, I finally got some 18650 batteries and a charger. So far I really like these. Basically they replace two CR123's (a little bigger in diameter and voltage so your flashlight needs to work with it).
Right now I'm using a Fenix TK21. With a max of 468 lumens, a nasty strobe, and other settings plus running on one 18650 this combo rocks.
It shouldn't be a surprise to hear that you can pick up this combo at Sawtooth Tactical. I've got a demo light you can check out in the store.
BTW, I've got some rechargeable CR123's on the way for those devices that use a single cell or that can't handle the 18650.