I have an MR920 Combat with an optic cut, so the G19 as opposed to the G17 version. I can give you my experience.
I’ve never had an issue with inadvertently dropping magazines, though I had read that as well. The magazine release is somewhat more extended than a standard Glock magazine release, but again it wasn’t an issue for me.
As for availability of parts, while the slide and frame are unique, the majority of parts can be replaced with factory Glock parts if need be, so I wouldn’t be overly worried about finding parts now or in the future.
The bigger question here is whether it is worth the premium over a standard Glock. I can only answer for myself and I think the answer depends on a few factors. The first factor is price. Around me a Glock MOS pre-tax is $620 . I’m using the MOS variant because the Elite comes with an optics cut. If I could get a Shadow Systems MR920 or DR920 for ~$850 or less, for me the price premium is worth it (and if you shop on gun.deals you can find an MR929 for well below that, the DR920 is newer and harder to find on sale). At what the standard MSRP is for these pistols, I would say no the premium isn’t worth it for me. The other purchasing factors are whether you plan to mount a RDS and have serious concerns about the Glock MOS optic mount system and whether the custom frame really makes a difference for you personally. Answers of yes to those questions make the 920 more appealing.
Why I put a price ceiling is I found next to my Gen 5 MOS Glocks there are diminishing returns. I didn’t get an MR920 Elite because serrations on the top of the slide do nothing for me (I would use the optic or rear sight to do one handed manipulations) and neither do the window cuts on the slide (I see them as an ingress point for debris). I can shoot my MR920 faster than my Glock and I like the way it fits in my hand (the backstrap system is also interesting as it does fundamentally change how the pistol points rather than just increase grip circumference). But is it a drastic difference? Honestly, no. That’s why I think at a certain price premium it makes sense, but not passed that.
I will also comment that the 920 series is based on a Gen 4 Glock. My experience in owning Gen 3, 4, and 5 Glocks and the MOS models of the last is that the Gen 5 Glock is both more reliable, more robust, and has a nicer trigger/is easier to shoot well. I wish that generation had been the basis for the 920, but it wasn’t. My 920 did have to go back to Shadow Systems due to sporadic failures to chamber a round. Shadow Systems paid shipping both ways and fixed it so quickly that I had the pistol back in a week from when I sent it out. The pistol has not had malfunctions since. I used that pistol in a two day course with Kyle Lamb where I was shooting under vehicles one handed with the pistol and the shooting bay had a sand base. The pistol was very sandy and continued to function, even when I could feel the grit in retracting the slide and pressing the trigger. I still carry and have confidence in the pistol, but I do think the stock Gen 5 MOS is more reliable.
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