I have a love hate relationship with them. I'd put it mostly down to the human in the equation though.
I have 2 of the 190 metal mags and one 130 plastic.
Every so often they develop serious feeding problems, but this seems to be as much to do with them getting dirty (again the sandpit seems to be my mag killer, so I need to look after them better), over filling them causes the springs to bind. The other issue seems to be bb related, the zero one bbs I bought 10 bags of are sh!t. They seem to be less slippery than the Devil blasters, (the surface is a bit more matt) this seems to cause them to bind in the tube which stops them feeding properly.