The bottom line: The Black Diamond ReVolt is an excellent, comfortable, and fully-featured headlamp that has everything you could possibly need with a headlamp. With optional rechargeable batteries, it's a great reliable choice for both casual and serious adventurers. However, expect significantly lowered performance with the rechargeables compared to normal alkaline batteries.
The Black Diamond ReVolt is a 300-lumen headlamp with 3 modes: bright/narrow, wide, and red. At over 3 ounces, it's not the lightest model - but for all but the most hardcore ultralighters, this is quite good. And within those 3 ounces, you get a wide array of features:
Let's talk about the adjustable brightness, which is super cool. By holding down the button, the LEDs will slowly brighten and dim until you let go of the light. This lets you dim the light to exactly whatever brightness you need, which is really useful and unique. In practice, I've found that I don't use it a ton, but it's still fun to play with.
The rechargeable batteries (included in the box) are another great feature, and in fact are why I bought the ReVolt. I hate buying (and throwing away) batteries, but I don't often do multi-day trips, so the rechargeable batteries are ideal for me. However, the ReVolt also accepts normal AAA batteries, so you can carry backup batteries. The rechargeable batteries can be charged in the headlamp by a micro-USB port, and you can view the charge status via an indicator LED. Note that you can only charge Black Diamond batteries through the USB - so don't try to charge a different brand's batteries in the headlamp.
Unfortunately, the rechargeable batteries don't have quite as much juice as the alkalines, so you'll see a drop in battery life, and a drop from 300 to 175 lumens at the brightest setting. This is pretty significant, but just about any headlamp is going to suffer from this difference. If you need those extra lumens, then you can either use the ReVolt with alkalines, or carry a backup headlamp with alkalines. Of course, you can also just use rechargeable batteries with a regular cheap headlamp too for the same effect, minus USB recharging.
If you don't need/want rechargeable batteries - or if you need longer battery life - take a look at the BD Spot or Storm, which are very similar in specs to the ReVolt but without rechargeable batteries.
I've seen some complaints of the lamp dimming quickly as the battery drains. I noticed this to some degree, but I still see the light output as usable. It seems that across the board headlamp manufacturers inflate their numbers for lumens and battery life a bit, and it's just something that consumers should expect at this point. I can run trails with the ReVolt when the battery is low, which should give you an indication of the brightness. Others might see this as an issue, but I find that it works well for me. As long as you know what to expect, you will be satisfied.
In terms of fit: the ReVolt is quite comfortable and fairly easy to adjust. The strap is just the right amount of elastic. It wobbles a bit when wearing tilted down over a hat and running hard, but anything less than that and it's sturdy as a rock. It's IPX8 waterproofed, so no worrying about hard rainstorms.
There's one more awesome feature that's almost hidden - BD call it "PowerTap." When the headlamp is already on, you can tap the right side of the headlamp gently and it will go to full brightness. Tap again, and you're back to low power. It's incredibly useful so you don't have to fuss with holding down the button and finding the right mode - just tap. I use this feature all the time.
The ReVolt is a bit more expensive than comparable models such as the Storm and the Spot, but taking into account the rechargeable batteries the price is reasonable in my opinion. It's heavier than ultralight lamps like the Petzl e+LITE by far, but also has more features and better battery life. My only real complaint is that there are so many types of button presses that it's hard to memorize how to use the headlamp! The headlamp is so complex it comes with a cheat sheet of actions and modes. Two presses to switch mode, three for strobe, 3-second press for red...it is a lot to remember. But one good thing is that all of BD's headlamps seem to use the same gestures.
So in conclusion - the Black Diamond ReVolt is a fantastic headlamp that I would strongly recommend to the majority of hikers and adventurers. It has all the features one could possibly need in a compact rechargeable package. Though the rechargeable batteries don't perform as well as alkaline batteries, the lamp is bright enough and lasts long enough for my uses. It's earned a permanent spot in my pack.