You should take a look at the J Point reflex, while it's not 100% perfection, it very serviceable and you really don't notice the flaws when you're actually using it.
Biggest flaw is that it's an always on sight with only 4 automatically selected levels of illumination. This means that it will "flare" at most indoor ranges and the dot will appear a bit fuzzy. Another minor flaw is that the lens is acrylic instead of glass and it will exhibit some minor optical defects.
Now for the postitives. One is that it's the lightest reflex on the market because of that acrylic lens and the polymer case. This means that on a reciprocating slide on a semi automatic it will produces less stress on the mount than any other reflex sight, so the chances of the mount failing are reduced. Second, the entire electronic package is enclosed within a solid block of molded epoxy, so it can take a licking and keep on ticking. Third, it's the only reflex sight I've found that has a rear sight notch molded into the case, so it can be co-witness in the event of a battery failure. However, to truly be used co-witness and have the elevation be correct will require the installation of a custom height front sight. I wouldn't bother with that, a bit of practice at the range using this feature will teach you how much "low" to offset your aim to get decent "combat" accuracy. One big plus of that notch is that it makes it easy to "find" the dot by "iron sighting" the gun initially and then using the dot when it pops into the window.
BTW, I currently have 3 S&W revolvers equipped with J Points, so you can say that I'm a fan of this sight. At some point I intend to mount one on my carry gun, a Sig P239, however I'll have to wait until I can either find a spare slide or another P239 at the right price.
As for why a spare slide, in my area all of the gunsmiths recomend that the mounting plate for the sight be permanently fixed to the slide with epoxy. They've seen too many mounting plates fail by either cracking at the dovetail or just open up the dovetail cut until it pops free. However, I haven't found one that actually has tried the J Point, most of the problems have been with the heavier Burris Fastfire or Doctor. So, I plan on dedicatin a slide to the J Point and retain an iron sighted spare.
BTW, J Point specifically mentions Bowie Tactical as one source for a melted in mount, so I would certainly give Bowie a call and see what they have to say about the J Point on your M&P, I suspect they have some experience with this sight. One big plus for a melted in mount is that it will improve the co-witness accuracy. Unfortunately this can't be done on a Sig, it interferes with the firing pin safety.
Now a couple of notes on shooting with a reflex sight. First, if you wear glasses, use the area of your glasses that are corrected for distance vision. The distance for the apparent image for the dot is about 50 yards out, so using near vision correction will just result in a huge fuzzy dot. I also don't think that the 8 MOA dot is really needed. Yeah, it will be easier to see but I've had no problems at all running speed drills with the 4 MOA dot and it does make it a lot easier to use for long range shooting.
BTW, if your interested in seeing my trio, check the following link. I've bench rested all 3 at 2 1/2 inch or less at 35 yards.
http://smith-wessonforum.com/lounge/155300-purchased-2-more-j-point-reflex-sights-range-report.html
