I had one for my Yugo 59/66A1 which also wore a Tapco poly stock. I experienced misfeeds often with the mag as it came out of the package, but all it took was to look at how the cartridges were feeding into the gun and see where the mag needed a bit of file work to feed properly. After just a little tweaking it worked perfectly, FMJ or HP, slow fire or mag dumps.
I will be the first to say that needing to "fix" a product to work right out of the box is a buzzkill, but the fact is the SKS is not an AK. Its hard to make a detachable magazine (that it was never designed to use) that will fit in, and function with all the different SKS rifles produced all over the world, right out of the box.
Its only $20. I would buy another one or more if I still had an SKS, for what its worth.