It seems like King Arms offers the best, cheap alternative to TM or CA mags.
They do have minor issues (in my case at least) that can be fixed.
My KA M16 mags didn't always lock in the magwell (TM M4A1) because the hole where the mag release is supposed to go wasn't perfectly straight. I had to file them a bit.
My KA G36 mags were too tight (CA G36), I had to file the 4 sides of the part that goes in the magwell to make them easier to insert and release.