I finally got around to putting a red dot sight on my M&P-15. It is a Vortex Crossfire, with a 2 MOA dot. My 54 year old eyes are not quite as sharp as they were when I was in the Marines, 35 years ago, but the red dot is certainly "faster" than my irons.
It has a lower 1/3 co-witness, that is cool because when I flip up the rear sight to use the irons, and lower my face a bit to get them lined up, the dot comes down and sits right on top of the FSP. If I flip the rear down and just use the dot, then the dot goes up just a tad to sit "above" the FSP. I like it...a lot!
I do a 50 yard zero with the .223/5.56mm, that way I am about 1.5" high at 100, about 1.5" low at 200, and about 9" low at 300...that is for 55-62 grain loads, which is about all I stock. It works for me, and I was able to hit the 12" steel gong, that my gun club has set up, 7 out of 10 times. Good enough for me.
I like the Vortex red dot, and for the price (149$), I recommend it.