"I have had these for a few months along side the PMAGs as well. So if you're looking for an opinion on which to get, I'll go a bit more into detail to possibly help decide. I am using the stock G&P bolt and nozzle for those curious.
(keep in mind these are STANAG styled):
STANAG: heavy and sleek in hand, feels great if you like the traditional look.
PMAG: feels natural in hand, but not as heavy as the STANAG, but comes with a nice, tight base plate that covers the fill valve.
STANAG: has pins to punch out, but from what I've seen so far I don't need to take them apart, just keep the seal on the bottom lubed.
PMAG: the unholy Christ of magazines. On the bottom it has one long screw, once this is out, the magazine will split into three pieces. there is a metal rod to hold the spring in place for the follower, and o-rings on the magazine pieces to ensure a good seal. The problem with this though, is that even if you oil them well, they can still leak, and you may even have to replace them! Not only this, but re-assembling is just of a nightmare, having to make sure the spring isn't caught and the bar isn't in the way. It's an alright fit, so you'll either need some help in order to make sure it's put back together correctly, or get lucky and do it yourself.
STANAG: The fill valve is on the back, which is fine, but the valve itself is rather long, so long that you could put at least THREE(3) o-rings on the valve to cover it. This means that it may spray a bit more gas, as well as not coming with o-rings(it wouldn't make a difference anyway). Whether it's less gas efficient or not I'm not 100% sure, but I'm skeptical with it.
PMAG: done perfectly, has a o-ring on the valve, shallow, fills perfectly. it even has a base-plate to cover it.
Final verdict/pros & cons
The overall build is nice, I personally prefer the look of these over PMAGs, but they were sold out at the time. The fill valves may just be my problem, but it's a simple thing to look over considering how nice these are. All-in-all, I would get these mags over the PMAGs.
cheaper than the PMAGs
holds more rounds
feels better in hand (personally)
not much need for take down
fill valve on the back.
Hope this has helped those who are unsure!