Two Fenix headlamps in service for over a year now. I have a couple of neighbors, Deputy Sheriffs, each living within waving distance of our house in town. They have been using the flashlights with the 18650 rechargeable batteries for a long time, and highly recommended them, so I started looking at all of the high end headlamps that utilized them. The Fenix brand has proven to be an excellent performer. Major improvements from the carbide headlamps that I first started out with my Grandfather.
18650 rechargeable batteries have changed the game for me in outdoor lighting. Lots of choices in battery capacity within the 18650 size. Started out with the 2400mAh batteries that came with the headlamp packages. Currently running the 3400mAh. The 18650 battery class is only slightly larger than AA. Can be recharged by house charger, 12V vehicle or boat, or USB. I carry a spare as a backup, but never have to replace the battery, unless we are frogging and utilize the high powered output choices a lot. One battery charge will last a week during my normal tripping use. I really like the USB choice, because I always carry a large capacity back-up battery for our phone recharging.
Turtle is correct that with external light sources, the brighter the light, the more disconnected with nature you are. But here in Louisiana, sometimes too dim of a light, does not alert you to the presence of certain forms of "nature" that can be very detrimental to your health.