I've got one. They're a blast. No problems in my WASR-10. Naturally, it's heavier than your standard 30 or 40 rd mag, but it isn't particularly bad.
If I ever get around to buying a Kalashnikov, I'll probably also buy one or two of those drums. As is, my 7.62x39 carbine is a Simonov. Two, actually: One Russki, one ChiCom. The Russki is by far the better quality, but I got the ChiCom plus 1000 rds Norinco steel-core ammo for $150, so I can't complain too loudly.