I have a Beta C-Mag that I run occasionally, it's fine if you've lubed it. Some friends own Surefire 60's and 100's which have run well in a couple of guns that we've tried them in. Earlier this summer we ran the crap outta a SF60 in my M16 and had a blast breaking in a 20" upper.
I have a couple of the Surefire 60-rd mags - they work great. I don't use 'em much, but they do work well. The top couple of inches of the mag look just like a standard mag so it can fit in the mag well. If the zombies ever get past the claymores and the aiming stakes, well, I got my 60-rd magazines...