The Streamlight ProTac HL is a 600 lumen output, C4 cree LED light powered by 2 CR123 batteries that I've had for a few months now; here's a quick summary of what I've found:
Pros:
-600 lumen output on high. It's VERY bright; see video below for the demonstration...
-"Aircraft grade" aluminum body
-Fits all 1'' light mounts
-Good checkering on the body for a good grip as a handheld light
-Scratch resistant lens
-The tail switch can be pressed for momentary on and clicked for constant on.
-Comes with a clip should you want to clip it on your belt as well as a nylon 'holster' with belt loops.
-Claimed 803 foot beam distance (I haven't been able to test this...)
-Programable for 3 modes (high/strobe/low, high only, low/high)
-O-rings on tail cap and lens (waterproof to 1m for 30 mins)
-Durable. I used this light on pump shotguns with no impact on performance
-Can be found most places under $75
-Limited lifetime warranty
-The finish is very even and it feels solid and well made in your hand.
Cons:
-The bezel is larger than most tac lights so if you're looking to use one of the red lens covers you have already it's likely not going to fit.
-Made in China
-In high output mode the battery life is 1.25 hours but according to their chart the output starts to taper off around 50 mins.
All in all I really like the light and have ordered a second in fact. I'm not sure it can be beat at its' price point in terms of brightness and durability.
Here's a video with some shooting footage (mounted on an AR), a low light illumination comparison to the 104 lumen Maglite & 200 lumen Surefire 6PX Tactical, and a discussion of the pros and cons of the light:
Streamlight ProTac HD Video Demonstration & Review Link