I find my Beta-Mag to be very light, when empty. Of course, it is a little heavier with 100 rounds of 5.56 in it, but not appreciably so. It is, in fact, very well-balanced and ergonomic in my AR-15.
I'm seriously considering getting one for the M14. Now, that's going to be a mite bit heavy. But, from a bench, hip, or bipod, why the heck not.
I consider Beta-Mags a novelty item, and enjoy having them as such. Anything that puts a burr up the butt of an Obama'ite, suits me just fine. I will use my P-Mags as my predominant SHTF mag, though, and they are pre-loaded for such a contingency.
But I still endorse the Beta-Mag unequivocally. It's a ton o' fun.