Don't really see the point to them, myself.
IMO, between the Magpul Pmag, regular GI style with an anti-tilt follower, and the TangoDown ARC, there's something in there for every budget and all are very reliable. Unless the Tapco was far cheaper than the $13 or so for the Pmags and GI w/ follower, there's not much point to it over one of the others and if it were that cheap, I'd have to wonder about the quality.
Kind of a catch-22... the market is already well served by good companies with good products at good prices; a newcomer has to offer something not offered already (either new features or better prices), and those would invariably be seen with suspicion.