Are they reliable or junk? Anyone with first hand reviews / experiences?

Thank you.

They need to make a slab side version.

I've never used the 20s, but my 10s and 30s have been perfectly reliable. Several folks here have Tapco 20s and I don't recall seeing any complaints.

My experiences with one 10rd and several 30rd mags have all been favorable.  I can't imagine why the 20rd mags would perform differently.  While not my favorite magazine compared to steel, they do work well and are readily available.  And if you're looking for 20rd mags, they may be about the only ones you'll easily find.  There are some 20rd combloc steel "tanker" mags, but these may be difficult to find.  I've also seen a good DIY video for cutting down Magpul 30rd mags if you're game for a hobby project.

I have three of the Korean twenty rounders.  They work perfect in my WASR-10.  No problems at all with them.