I get questions about this quite often. So at the risk of offending somebody, here is why I think the MOS system is a poorly thought out Compromise and should not be a top choice for those wanting to get into the red dot world of pistols.
The MOS was developed for a single purpose, and excellence for the end-user was not the reason. It was developed so Gaston Glock could sell red dot capable slides to all the cheap bastards that are too cheap to buy a Trijicon RMR or a Leupold Deltapoint Pro. They will glom on to trash like Burris Fastfire and want to install it on their slide.
The MOS was designed to accept any and all red dots and thus it is an exercise in compromise...period. This may not be a concern for GSSF or other pistol games, but those wishing to carry for more serious reasons, here are the problems I have seen with the system.
Problems with the MOS -
1). The Compromise requires a taller mount of the red dot than is necessary causing a much more height-over-bore problem than is desirable. The taller the sights the more variance between red dot and barrel...the more difficulty in shooting well.
2). The Compromise weakens the slide. The cut for the MOS is far lower than I think is safe and I expect in short order, volume shooters will have problems in the extractor area of the slide where it is extremely thin.
3). The Compromise requires an interface plate which is mounted on the slide, very securely...BUT the red dot is mounted onto the plate, which is very thing and gives very little purchase (that meas a secure grip) for the red dot. The loss of red dots (like in flying off the slide) is common with MOS slides...specially with hot ammo and with the heavier calibers. Hell...even the S&W CORE is a better design!
4). Dealer price for a standard Glock 17 is $440....for a MOS it is $540...or $100 more. Milling the slide properly for a tier one red dot like the RMR or the DP Pro, with a lower, and far more secure installation, without all the problems associated with the Compromise, is only $30 more than the difference. So unless somebody simply wants to spend the money for The Compromise, I can think of no good reason to do so.
5). And the cost of a slide, of greater quality than the Glock factory, built from the ground up to accept a Tier one red dot, will be less than the cost of a new Compromise pistol.
So unless the goal is to buy poor gear in order to use a poorer red dot, I am at a loss about what the advantage of The Compromise might be. There are far better, and actually more cost-effective, options available.