So, I've been using an ArmyTek 18650 headlamp here in A'Stan for the past few months.  I just have a couple of 18650 (unprotected, but I have an appropriate charger) 3400mAh 3.7V batteries.  I've been using this headlamp nightly for 1-2 hours and in the morning for a few minutes before I get my room lights on; a couple times I've left my light on all day when I forget to turn it off.  I've been using the same battery for three months without a charge...pretty freaking impressive.  For home use, 18650's are a no-brainer.  For backpacking, I just don't spend three months on a single trip to justify the extra weight, but regardless, I'm impressed with the capacity of the 18650 batteries.  The ArmyTek headlamp is excellent as well...

ROCK6

I think for high output or long runtime LED lights 18650's are the way to go.

For the most part I agree.  However, I'm still rocking a much lighter Zebra single AA headlamp as my primary for backpacking.  Even with the lighter lithium battery, for normal "backpacking" use, I can go over a week for my illumination needs.  The larger 18650 is something I would consider on a six month expedition.  I won't get to experiment until next summer, but I want to see how well my solar panel (Suntactics-14) will charge the higher capacity 18650's (I have a single 5V/USB charger).  If I could get up to an 80% charge, a couple batteries would likely last you a decade of "normal" use.  This is also why I prefer the unprotected batteries as they have less resistance.  Regardless, while the lights are a little larger, the capacity is far greater and the single cell is more efficient.

ROCK6