I bought my Polytac primarily because it was cheap a while back and I've been using if for a couple years now; here's a quick summary of my findings thus far.
-Cost. The light can be had for under $35 shipped.
-Uses relatively common CR123 batteries.
-Fits in common 1'' mounts.
-It works. So far I all the Polytacs I own have kept on ticking even with use on rifles and shotguns.
-130 C4 LED output. It's a nice white light with a good peripheral cone.
-Has o rings to seal out water on both the tail cap and lens cap.
-Lightweight. Weighs 4.1oz with batteries.
-Comes in black, coyote, and bright yellow.
-1 of the 3 I lights I have flickers slightly under recoil (can be seen at the beginning of the video below). Is it a huge issue for SD/HD? Maybe. I really doubt I'd ever shoot in the dark with the light off in a HD/SD situation but for use with night vision devices it's a different story.
-It's not as bright as some lights on the market.
Here's a video with some shooting/testing, a night time comparison between a 104 lumen Maglite and a Surefire 6PX Tactical, followed by a discussion of the light:
Streamlight Polytac HD Video Review Link