- Rugged/Waterproof/Reliable
Bright enough to be useful in supplementing strobes for cave photos, or even be used by itself for more confined shots. Also, able to dim to a reasonable level for hanging out/eating, etc.
Options for both wide (right 2 leds), narrow (left), and combined beams (using the rotary switch)
Neutral to warm color rather than the standard blueish led's
Repairable/upgradeable without too much trouble - able to be visually inspected while in a cave.
Good efficiency and therefore good battery life
Cost around $100...ended up costing around $200 including the fancy battery and various shipping costs from all over the world
As compact as possible while meeting the above goals
I pretty much achieved all these goals except for the cost and possibly the low brightness not being quite as dim as I'd like. Also, I would always like a less bulky headlamp, although it's not as bad as some I've seen. It seems to be pretty waterproof from my limited testing. It's pretty much a short section of 3"x2" Aluminum tubing with an aluminum plate on the back and thick acrylic on the front, all sandwiched together with 4 machine screws and some gaskets.
It puts out about 700-750 theoretical lumens on it's highest setting, and about 30 lumens on the lowest. On high the LED's are being driven over spec, but they're mounted on a pretty big chunk of aluminum so they should be fine as long as I don't keep it on high for a whole trip. I haven't actually done real life testing for the runtime on the battery I have but it should get about 5 hours on high, 24 hours on medium, and 4 days on low according to my calculations. It definitely heats up pretty well when on high, although it seemed to have been on high for a good chunk of time during the drive out to a cave the other day (whoops) and though it was hot, the thermal protection didn't seem to have kicked in. The li-ion battery I'm using is a little overkill so I'll probably swap that out for one that's 1/2 the size/capacity/battery life. Still plenty of juice for just about any trip.
For those who are interested in the more technical stuff it uses 4 Cree XP-E R2 bin Led's and the hipFlex driver from TaskLED. I can't praise this driver board enough. Many of the settings are programmable, like max output, thermal protection temp, warning voltage, cutoff voltage, and you can mess with the lighting modes. I was going to get a cheaper driver and just make it work but I'm glad I went with this as it allows for a lot of flexibility should I want to change anything somewhere down the line. Also, George from TaskLED offers some of the best product support I've ever experienced. (and I needed the help!) The headlamp has some level of built in redundancy in the fact that it has 2 separate and independent strings of LED's. If one led goes, disabling a series string, (probably not the most likely failure mode, although it could happen) I can switch from wide to narrow beam, or vise versa to resolve the issue.
Tried it out in a cave for the first time the other day and was very pleased with how it worked. I don't think I'll really trust it fully till I have a few more lengthy trips with it. The big button is super easy to manipulate and the rotary switch is a great way to switch between beams. Overall, I couldn't be happier with how it turned out.
If you have the time and the inclination I would definitely recommend building a headlamp to anyone leaning in that direction. I had only the most basic understanding of electronics (like...what I learned in middle school) when I started, and still only understand just enough. If it were't for the Internet and the various light modding/building communities out there I never could have conceived of doing this. 
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