i just sprung for one, 6X Wolfhound. i also have the Primary Arms 5X prism scope. the Sightmark reticle center dot is smaller than the PA 5X (0.5MOA vs 1.0MOA), which i prefer. note: i'm not displeased with the PA 5X at all, sofar it's simple and dependable. the bigger reticle center dot works fine for aiming at silhouettes at range, but for trying to aim at a 1" black dot at 100yds, it covers the dot.
The Sightmark 6X comes with a kill-flash which threads on the front. i was able to use a Vortex Bushwacker #6 flip-up lens cover on the objective, and a Vortex Bushwacker #4 on the eye piece, instead of the supplied bikini lens cover, since i had those on-hand. will be trying out Butler Creek flip-ups also to see what actually works, fits, stays put, stays shut when supposed to. i like to use the BC Blizzard (clear) on the eye piece whenever it works, but the fit is hit-and-miss.
haven't taken the Sightmark to the range yet to sight it in, will post my observations (if any) after it do. i did sight it in the length of the hallway of my house into the garage, the picture is clear, no complaints. audible click-stops on the windage, elevation adjustments, i use a nickel for the slots. 1/4" MOA per click.
the rail attachment is about 2-9/16" in length, and i'm going to try to mount it on a picatinny QD adapter as soon as i can find one of the right size, that doesn't raise it too much nor use too much rail real-estate. as others have commented, wished the Sightmark Wolfhound had the ACOG foot so that i could take advantage of the many ACOG QD mounts readily available.
the Sightmark is bulkier, and slight longer than the PA 5X, but the clamp-on mount is pretty low-profile, so as-is, it doesn't make the AR package any taller.