I have personally used the King Arms mags and I can say that they have held up for about 2 years. (I usually play about 1-3 times a month.) The major grip I do have about them is that they don't feed any weight above .25g very well. My 20-25 RPS guns will generally misfeed .28g's for me, but feed .25g flawlessly. I've dropped a number of those mags and they've held up okay. I've since then switch over to KWA K120's, which I find to be much superior in all respects.
As far as the BETA Project magazines go, I've never personally used them, but I know guys that do. All I've heard are mixed reviews from magazines not feeding to feeding up to .30g without issues.
Hope that helps.