I've never heard anything good about them. As to say, everything I've heard has been bad. Real bad.
I've shot a fair amount of ammo through .223 C-Mags. They do work. They're not milspec, though, so I wouldn't expect them to take a real pounding. I've heard of guys breaking them, but have never seen it happen. Spare parts are supposedly relatively easy to come by. A friend of mine hoses 5.56 through them in an HK33 by the metric ton. So they do work.
Not bad, but the feed tower can crack (it is replacable) if you drop it/are rough with it. The last dozen rounds can be a motherf**ker to get in w/o a loader. Makes the entire firearm till to the left. If you are right handed, you can support the 'drum' with the crook of your left arm. If you are a lefty...you're sol!
The weight is balanced on both sides, but, on an AR, the mag catch is hard to get at. Makes the firearm rather bulbous and awkward in the middle and lastly, it is not the most durable item out there.
Neither of these magazines are tough enough for duty (LE/Mil) useage.
If you have the money to buy one, as a range toy, why not?
If you are actually looking at these for some sort of duty usage, just buy 30 round magazines. They're much cheaper and will take more abuse.