I've got one of the plastic 30-rounders that came with my SKS. Take my advice, stay away from the plastic ones. Avoid them like the plague. They are a brainchild of Satan. Is there any confusion regarding my feelings towards the plastic SKS mags? I should hope not. It's darn near impossible to reload with stripper clips, as the mag has to stay in the weapon to do so, and the bolt keeps wanting to come forward after the first stripper is fed into it. When I try to reload by hand, it starts getting tough to put any ammo into it after about 20 - 25 rounds. They don't want to seat properly. Also, the spring and follower like to stick about an inch or so down, causing the weapon to have feeding problems every so often.
As soon as I can afford it, I'll be buying one or two metal 30-rounders and a 10 rounder. I also have a flush-fit 5-round mag, but that thing is about as useless as, well, I think ya'll can guess what it's as useless as. (Art's grammaw may be reading this, so I'll keep it civil.)
Frank