Kytex shooting gear makes some really nice mag pouches. I have a few for Glock 17 mags, which I've had for about a year and have run through a number of classes. Compared to other mag pouches I've tried, they're inexpensive, they ship fast, and they can be clipped on without taking off your belt. They're still very secure despite the fact that they clip on.
What makes them really stand out is the fact that they're very compact, light, and thin, and made out of one single piece of kydex with no additional hardware like screws, etc, so you can fit more of them on a given area of belt line, and they hold very tight to the body for concealment with a loose shirt.
The only downside is that I would say that they are a little flimsier than some others just based on the fact that they're made entirely out of thin Kydex. One of mine developed a small crack from when one of my mags got hooked on something and was pulled hard away from my body. It was still usable and probably would have been okay with the application of some duct tape. Brian, the owner of Kytex, offered to send me a replacement. I only had to send him a picture of my cracked one; I didn't have to ship it back.
Also, the retention may loosen up a little over time since the retention comes from the form of the Kydex and there aren't any screws or rivets. If you are at all handy with a heat gun or blow dryer, that is easy to fix.
I probably would not use these in some kind of austere situation overseas where gear must be super durable because you don't know when you're going to get a resupply. But for any normal use, they're great and backed by great customer service.
Kyle Defoor is a fan of the AR-15 mag pouches as seen on his blog here.