Compact size, reliable performance and ruggedness. I'm a fan of the "the gun/knife/flashlight you have with you is better than the one you left at home" club, so MagLights and most StreamLights are out for me. I like the looks/size/beam of the SureFire 6P, but even though they have name recognition in tactical flashlights going for them, they just don't cut it for me. I'm clumsy. I drop things. SureFire bulbs are touchy. They don't like being dropped. I don't like paying $25 for a bulb because I dropped my light, so I have TACM3s. http://www.tacm3.com
They are approximately the same size as the surefire 6p and they all come standard with shock resistant bulbs. I mean, you can (and I have) throw these little babies on concrete as hard as you can and they still work. I'm also a big fan of flashlights with the on button on the tail cap or pigtail switch. It makes finding the switch a real no-brainer. The only thing I don't like about the TACM3s is they take those doggone 3v lithium batteries.
And those little photon lights are awfully handy critters to have around.