I got a few 20 round Tapco mags after reading good reviews at
http://www.sksboards.com/smf/index.php
I havnt had any problems with mine at all, when the gun runs empty the mag pushes up the bolt hold open just like the fixed mag.
Theirs a easy bolt mod that lets you insert or remove the magazine with the bolt open or closed.
In the past alot of detachable mags didnt work good or at all without some tweeking and it turned alotta people off to the idea but their are a few out now that seem to work pretty good for alot of people.