I recently purchased a Vortex VMX3-T magnifier and installed it on my M&P15.  The magnifier comes with a shim for 1/3 co witness, or you can run it without if you prefer absolute co witness.  When installing I did put blue loctite on the screws that secure the magnifier to the mount.  Today I took my Vortex VMX3-t magnifier out to the range.  Got everything dialed in and shot at 7-25-50 and 100 yards. I only used the magnifier at 50-100 yards. At first I was thinking "this thing doesn't really bring the target in that much closer", but after shooting with and without the magnifier it made a difference. My groups didn't shrink considerably, but it was a lot easier identifying the target and making faster, longer range shots. I also shot at the 7 and 25 yard distances with the magnifier flipped to the side and it didn't distract me or block my peripheral vision. I didn't really beat it up, but it performed great, the glass was crystal clear, and I really liked the flipped to the side mount. Once locked, it's not moving unless you press the button to release it.  I use the magnifier with the release button facing towards my Eotech, so that the magnifier flips to the right.  When flipped I did not experience any brass hitting the magnifier. All in all, I'm extremely happy with my purchase.

Thanks for the review!