This is just my actual experiance with drums ... I've got Beta-Mags in 5.56 for my AR & HK93 and 1 in 9mm for my HK MP5's (5, 5K & 5SD) ... In the last 8-10 years they mostly just sit on the shelf, which is why I haven't bothered to buy any of the new Magpuls.
Now, that being said, if I didn't have the 3 Beta-Mags, there's a good chance I'd buy one just for the cool factor.