I've had the Surefire X300U for a while now but never did a review; so here goes...
-500 lumens with a good balance of throw/spill
-Uses two 3V CR2 lithium battery.
-Comes with a 1913 and universal mount/insert so it'll fit most modern rail systems to include rifles
-It handles recoil well. I've used the light on my G20 with hot ammo as well as on an AK with 0 issues from the recoil
-Rocker switch can be used for temporary and constant on modes
-1.5 hours of constant on. The old battery life vs lumens issue is at play here.
-It's extremely tight when mounted on some guns. This could be a pro for some but, on my Glocks for example, the light is seemingly stuck on the gun and I've had to tap it with a rubber mallet while pulling down the tabs before to get it off.
-Price. No getting around it---it's expensive. It generally runs around $210-240.
It's an excellent all around weapon light. It's versatile, bright, and durable. The big question is whether or not it's worth the money. Only you can answer that ultimately I suppose.
Here's a video showing the light in use on several guns, a comparison with some different lights, some night shooting, and a discussion of the pros/cons of the light: