I saw these the other day.
Here is my $0.02. They cost about 2x as much as PMags, and what is the actual benefit of the front of the mag sliding down for loading and unloading. Is it really that much faster? Is the edge sharp? Does the mag ever open accidentally and spill ammo out?
If you are ever in a situation where you needed to really quickly load a mag, don't you think you would probably have a couple of extra 30 rounders already loaded in your pockets?
I think I am gonna stick with these...
http://www.youtube.com/watch?v=UCFXcOOb4jc