I have some for my AR. I like them just fine. They would be expensive for plastic or for metal, but if metal, likely much heavier.
They hold their tension just fine. I found that best use for them is in the form of bipod or rested shooting where you don't wan't to have to reload 20 rounders time after time and a 30 rounder is too long. For example, we sometimes do dueling trees at 100 and 200 yards and the Beta C is great for that.
They are heavy for carry but would not be bad out of a vehicle.
For how many rounds are they reliable? Gee, I don't know. 21 mags is 2100 rounds and that would be 70 with a 30 rounder or 100.5 with a 20 rounder.
I probably have 3000 rounds through one of mine. I know that doesn't seem like much, but it is a good bit of shooting.