I like the MAG mags, they are cheap, light, and seem to be reliable. They are 100 rnds, but seem to work better if you don't stuff them full. I usually only put in 80. Out of the 16 that I have purchased only 1 has gone down. At 7 bucks a piece(when you buy an 8 pack) the Price is right also.
On the downside, they are plastic(so less realistic) and the last 3 or 4 BB's don't feed. So with every mag change you lose 3 or 4 BB's.
Just my 2 cents,
Charlie