I've had the HSP WML for about a month now and have been running in on a couple long guns. Here's what I've found so far...
-Cost. The light can be had for around $120 shipped.
-Uses one 3V CR2 lithium battery.
-Easy on/off mounting. Very secure once mounted
-It handles recoil well. No issues on the 12ga pump shotguns
-Momentary on/off only. I generally prefer simple on/off controls on defensive lights but this may be a "con" for some folks.
-200 lumen output
-Has two methods of preventing it from accidently being activated (1. twist the bezel or put up the hinge on the back of the light to prevent it from being hit by gear/ect...)
-Lightweight. 3oz with battery (includes mount)
-1.5 hour constant run time.
-Cost. Comes in at around $109
-Only comes in black (due to it being the strongest polymer)
Here's a video showing some day/night shooting, a comparison with a 104 lumen LED Maglite and a Surefire X300U, a deomonstration of the mounting system, and a discussion of the pros and cons of the light: