I have one on a complete Aero Precision M4e1 upper that I ordered during a sale last summer. I ordered the upper with a 15" S-one hand guard on a 16" barrel. I had a DD fixed front sight, mounted on the front pic-rail to lower 1/3rd co-witness with a EXPS-2 (got it new on an amazing sale) and a flip up griffin m2 rear sight. Loved the rail until about a month ago when I checked the co-witness with the irons and the eotech prior to an outdoor range session and noticed the front sight was canted to the right. I found the locking mechanism holding the rail in place had walked just enough overtime to become loose enough to rotate from side to side with the amount of force needed to tighten the sling on the rifle. I had not noticed any play in the hand guard until that point and I dry-fire practice anywhere from 3-6 days a week for 15-30 minutes a session.
Due to being a college student with limited tools, I was able to lock the hand guard down to where it will not walk, however I cannot index the rail to the receiver to trust running a set of backup iron sights. This all took place within about 600-700 rounds through the rifle since purchasing it last summer without playing with the locking mechanism until I noticed the problem. (college student ammo budget, not a long-term eval)
All-in-all:
I think the rail is very comfortable
I enjoy how light weight the rail is
It does everything I need it to do (I don't have any lasers so not worried about that)
However, the locking mechanism slipped on my upper I don't trust it to host any backup aiming system.
*sample size of one*