I had one for several years and sold it a few months ago.
I had the 8" version, it was ok.
The bolt hold open was kinda iffy on it, if I had a decently used magazine sometimes it would fail to hold back and the bolt started chewing on the bolt catch a little (steel vs aluminum). I contemplated removing it completely and using it like an AK.
I swapped the spring out in the paddle for a weaker one so more mags would work with the bolt hold open.
It wasnt terribly accurate, mine shot about 3-4" groups at 25 yards. I even tried working up handloads for it with different bullet weights and powder types but never came up with a really accurate combination.
The charging handle annoyed me because it was proprietary.
Never had any function problems with it though.
I sold it when I built an aero EPC, which IMO is the way to go for an AR style 9mm.
my EPC, scorpion, and KP9 are all much more accurate than the FX-9 was. They all shoot clover hole groups at 25 yards.