Just in time for Christmas my SGM Tactical mags showed up last week, here's what I've found...
-Lightweight polymer design
-Modular design. They can be cut along the ribs to make 10, 8, ect... round mags
-Even fully loaded they still balance well in the gun
-Reliably feed variety of loads
-Did I mention they give you 12+1 capability?
-Easy to disassemble/clean
-Cost. They can be purchased for around $50
-They lock up solidly in the gun
-They make firing from the prone awkward
-I've fired 96 rounds of bulk Walmart Winchester 8 shot loads---one time it didn't lock the bolt back. It only happened with that load so most likely it wasn't the mag but just noting it for others in case they experience it.
Here's a video with some shooting, a breakdown of the mag's properties, and a discussion of the pros and cons: