Great on a on a full sized xdm - Vortex Defender-XL 1x25mm Reflex Red Dot Sight

This is an excellent optic, the sight picture is second only to my eotech or Holosun 510C. It's not what I would use for CC but for a full sized pistol, competition shooting, bill drills I was able to easily and rapidly acquire. I have become somewhat of a Vortex fan over the last few years, Holosun disappoints me with warranty, durability, (threading ability to accept proper torque) and sight picture i.e the 507X2.

Downsides: It is big but hey that is why I bought it
I bought two, one was and older version that required backing a screw out to open the battery door. (Vortex tech support, which is excellent by the way said that was no longer required in later versions (which my second XL did not require the screw to open the battery door) I used the second as a canted backup sight for a DMR build.