I got these a while ago and I've run them through their paces. For one I prefer the polymer. My load out is pushing 20 pounds already, plus my 12 pound gun. And I don't even play big games (I use 8 mags and 3 pistol mags). I play outside in a very forested area with only a handful of people. Because of the lack of people, it's not a shoot and survive play style. It's more of hunting. You have a conflict maybe every 5 minutes, rather then every 5 seconds (on average), So sneaking is key. High caps have the bb's loose in an empty void, and rattle like crazy. This gives you away like none other, hence why I changed to mid caps. 110% silent. Also when in a firefight, you can shoot all 120 rounds holding the trigger. High caps I think shoot 30 rounds before you have to wind them up again.
Now loading these things is the only real problem. It requires a speed loader (that or pressing in 120 bbs one at a time) where as high cap you simply pour them in. Also you should fill up to 120, no more, however this mag will fit more. I believe I got 140 into one once. As good as extra bb's sound, trust me it's bad. Basically the bb's stack at in a zig zag lined right? When you force extras in, the bb's stack parallel to each other in the shaft. This makes them stuck. In other words, you will be shooting, only 6 bb's will come out, then the mag will jam. Once loose, shoot another 6 or so shots, then it's jammed again. The parallel bb's usually happen throughout the mag, so it'll jam until all bb's are gone. Now I've learned from experience a firm slap on the side or bottom of the mag is usually enough of a jolt to unjam it. So there's a solution. However a jamed mag will require to be unjammed (slapped) anywhere from 5-20 times during those 140ish bb's. Trust me in a fire fight, this is among one of the top three worst things that could happen to you. So it's very important that you load it to 120, or LESS. No more then that. It helps if you have a speed loader that is visably transparent, and has a measuring line (like a line for 60 bbs, fill it twice with that amount equals 120). I saw on evike tv that some company is making a shotgun looking speed loader specifically for midcaps, but have yet to see it be released yet (highly anticipating that).
On my JG M416, these mags fit the mag-well very tightly. To clarify what I mean, I have to mildly force them into the mag-well. Basically I push them half way (not that hard) then give them a good spank (not that hard) and they click in. Getting them out is some what of a tug, but not a horrible deal. While some don't prefer this, I prefer it over my hi caps that have at times fallen out of my gun due to sheer weight of the metal mag+400 bbs, and because it's not a snug fit. I've also customized the 8 mags I use all with magpuls, and 4 dual mag connectors (1 for a pair).
I do prefer the od, cause it adds to the camo of my gun. My gun is basic black, but i have od and tan rail covers (mixed) tan pistol grip, stock, etc. So it's not a nice paint job camo, but it's better in the woods then black, on black, with some black on it, under some black. Not as bad as a neon color in the woods, but being closer to camo is best.
Pull trigger through 120 rounds (vs hi cap 40 rounds before having to wind it)
Snug fit, not loose
Difficult to load (but not impossible)
Easy to overfill if not loaded properly
Overfilling causes overwhelming amounts of mag jams (see review for further details)