Work great on fixed stock rifles.
SHITTY on collapsible stock models.
Fixed stock rifles keep the magazine sitting fairly far back, in the same place, out of the way.
I don't object to the idea of having a magazine on the weapon at all, for many situations, it works out pretty well. The concept is not the problem.
The problem with almost every model I've seen when mounted on a collapsible stock is that it interferes with the charging handle and the adjustment latch. If your rifle is always fully extended, it may not be so much of a problem, otherwise, if you want it collapsed at any point in time, or to be able to adjust it freely, it simply doesn't work very well in practice.
I've seen several soldiers have FTFs while using a side stock pouch, when they yank the charging handle, and it hangs up on the straps, and the effect is the same as riding the charging handle. It simply does not have the force to go into battery. This of course, coupled with poorly trained immediate action drills causes double feeds, and all other manner of problem, eliminating all of the advantage that having a magazine with the rifle in the first place had.
In practical use, I simply find that they are not worth the potential problems they cause. They look cool and all, and it's nice to have a magazine every time you pick up the weapon, but there are too many ways for it to go wrong on a collapsible stock carbine.
Of course, like I said, it's very different on a fixed stock rifle, and when I build my A2/A4/SDM-R (haven't decided which yet!
) clone, I will probably use one.
~Augee
ETA: A lot of soldiers now put their magazine pouches around the handguards, CAA does make a product, as well as that nifty IDF mag holder, but PX's don't carry those, so soldiers generally don't worry about them.
Honestly, more than anything else, I simply don't have room on my handguards for a magazine, my sling runs along one side, a VFG on the bottom rail, and a light on the 3 O'clock, topped off with a PEQ2 doesn't leave much room for a magazine.