I first ventured into rigid plastic retention 'FASTMags' a few years ago and haven't looked back. I originally bought 4x Gen 2 replica FASTMags from RoyalTigerGear to go with my Flyye EDC HydPack and associated fast changeover Flyye PLB Belt rigs.
To begin with I mounted them opening face up across the PLB Belt I wore back to front across my chest, which was buckled up at the rear beneath the HydPack and attached to the D Rings on the front of its straps using a TacLink each side.
This worked reasonably well and I used this rig in various woodland and urban games without any real issues; FASTMags are excellent in terms of good retention, fluid reloading and easy of single handed magazine handling, also allowing the negating of a dump pouch once you get used to reloading and then depositing the empty mag in the position vacated by the fresh one.
This can be a real advantage for in game ammo management and avoiding losing your empties or having a big-ass dump pouch getting in the way like a gaping whale shark maw on your hip... which also avoids it scooping up half the forest when you're doing woodland games, and thus keeps you, your kit, and your mags and their feed openings, all a lot less messy.
After a few games of this setup I decided to take the plunge and see if they worked inverted. This proved to be an excellent method of speeding up reloads, particularly as I was using either a G&G F2000 or Real Sword QBZ97 with my STANAG M4 magazines at the time... if your bullpup main weapon's magazine well just happens to be right next to your chest rig mounted FASTMag, and the magazine is facing the same way as the outgoing empty, you can soon find yourself reloading in an obscenely quick fashion, usually meaning you don't even have to look at what you're doing as it all becomes intuitive and muscle memory based with everything being so close together anyway.
Since doing this experiment I can't recall a single case where the retention has failed, even with heavier metal shelled M4 midcaps or hicaps and running around at 'full tilt', the mags just don't shift until you wrench them out for a reload. Most. Impressive.
When it comes to the related 9mm pistol magazine pouches, however, I would highly advise against inverting them! If you're only tiptoeing around CQB sites you might get away with it, but with GBB pistol mags being about a quarter of the cost of their gun, or roughly a pound-per-round of storage as well as considerably heavier AND having a smaller retention contact surface they are more likely to fall out and get lost, damaged, or drop right on your foot as you kick it into the falling mag whilst running for cover. Yes. That happened to me personally. It bloody hurt. My foot broke the magazine's fall at least.
When I got my array of 9mm FASTMags from Ebairsoft they seemed a little loose, after one game day of using them facing upwards I thought they were liable to lose me some expensive pistol mags even then. Thus I bought a load of Neoprene strip and cut it down to fill both internal faces of the plastic clamshells. Since making this modification the pistol mags don't go anywhere until you want them to.
The whole FASTMag system is ideal for CQB or pistol/M4 midcap usage with fluid reloading and easy ammo management. Until I recently sold it to a friend, I had a 901 chestrig with a Maruzen Walther P99 on a cross draw SERPA Level 2 Retention holster and TWELVE magazines in my modified 9mm FASTMags around the whole left side panel across the front through to the right of the middle, allowing me to reload ridiculously fast with my supporting hand, taking out a fresh mag as I neared the end of the one fitted to the pistol, changing over in one fluid move, then depositing the empty in the vacated space.
The only downside I have found since buying more of the newer Gen 3 FASTMags is that (obviously in a cost and complexity saving measure) they have ditched off the usual snap-pop fasteners and fabric straps in favour of rubberised plastic ones.
Whilst these and the new clamshell design mean you can now stack outer M4 FASTMags onto Inner ones which then mount to the vest, or stack two 9mm FASTMags onto the vest mounted M4 ones, the durability of the fastening is far reduced in my experience.
The rubberise plastic is very prone to tearing, the retention or fitting of the mounting strap to the hooks on the bottom face of the FASTMags is not great, especially with movement and pistol magazine weight loading on them.
If you want to put a FASTMag rig together and leave it be they are fine for that; I merely glued the rubberised plastic tabs to their associated hooks. But obviously this means they are prone to tearing and lots of re-arrangement is just a no-go.
All in all I've had a very positive experience with FASTMags, although I would strongly advise you to buy the cheap Ebairsoft replicas rather than the £20 each per pistol magazine real deal! Unless your life genuinely depends on them.