Retention is excellent, right-side up. They can be mounted upside down, but I don't think a mag will stay. On facebook one of the Milsig owners I believe did not recommend running them upside down. I ran around a bit with them upside down and the mags do slip out a bit. The inner face of the back half has a small length of thick rubber that helps keep the mag in. The inner face of the front half is just smooth plastic. You can stack them back to front, but not on the thin sides. In the end, the Faz Mag works as designed and advertised.
These were CDN$17.95 per at milsig.ca, not sure what US pricing will be.