I have the X300 on my M4 carbine and it's turned out to be an excellent choice for that application. Reliability, durability, brightness, beam quality, fit and finish, etc., are all what we've come to expect from Surefire. That's not to say there hasn't been some room left for improvement, however, mostly with regards to the clamping mechanism.
Perhaps in an effort to maintain the low profile of the light, SF designed their clamp to squeeze the Pic rail vertically. IOW, the upper lip of the mount is pulled towards the lower lip of the mount. Getting the light to really feel secure on the rail generally requires snugging up the hex head rail screws a bit after the light is positioned on the weapon. This works well. The tradeoff, though, is that it's now virtually impossible to remove the light using the quick detach feature without once again loosening these screws (a reasonably significant consideration in light of the fact that you can't swap out the X300's batteries while it's mounted on a weapon).
The Streamlights, on the other hand, exert force on the rail transversely, drawing the left and right halves together. This setup may be an advantage in pistol mounted applications where the accessory rails tend to be shorter and often of a non-metallic construction.