I've got one of those for the Glock 30. It's not any less comfortable than my Galco Jackass, despite using a much harder leather. Comfort with these things seems to vary from person to person though.
Something to note about this particular model: It balances well with the dual mag carrier, however I personally find the carrier harder to conceal than the gun. Since Mak mags are so small it might not be a problem. It probably also wouldn't be an issue if you're wearing a thicker cover garment, like a coat, or if you're not real thin.
I'm pretty wiry and almost always wearing a suit jacket when I've got that holster on, so I've taken to using the blank and carrying a mag in my pocket. If I had it to do over again I would've gotten the single mag carrier instead. If you don't mind having the harness fairly tight, though, the blank seems to work fine.
In response to Edmond, I'd say mine holds the pistol very securely. The snap closure that you draw through on the FIST holsters works very well in my experience. When the gun is holstered, it's not going anywhere unless I want it to. It also feels like the grip-up orientation would be slightly better for retention than a typical horizontal shoulder holster, but that's simply conjecture on my part.
I hope you find the above info useful. Let me know if you have any other specific questions.
