My only experience with ProMags has been with steel magazines for the Beretta 92 and the Ruger Mini 14. Both were left loaded continuously for months at a time, and gave perfect function.
Based on user reviews I would be reluctant to buy their polymer AK mags.
I do have a half-dozen 7.62x39 Tapco polymer AK mags in 10, 20 and 30 round configurations. They are basically a good product that may require a little tweaking to perfect them.
They seem to be sized with the largest magazine opening in mind. The sides of several had to be sanded down before they would fit easily in my WASR.
I also noted that they worked great in warm weather, but got a little sluggish in cold weather. So I tore each of them down and sanded material off the sides of the follower, until the follower would drop freely up and down in the magazine by gravity alone. Some were fine & others needed a little sanding. Cleaning out the sanding grit and hosing the internals down with CRC Silicon Spray, completed the process. I have since run them all after being left loaded in the trunk, in 10 degree weather. They are run slick as can be now.
I realize some will grumble about having to work on a new magazine. I gave between 8-11 bucks for these and I don't mind tweaking them a little at that price. I'm sure glad I bought them now.
“Nine-tenths of tactics are certain, and taught in books: but the irrational tenth is like the kingfisher flashing across the pool, and that is the test of generals. It can only be ensured by instinct, sharpened by thought practicing the stroke so often that at the crisis it is as natural as a reflex.” ~ T. E. Lawrence