I've been using the new SGM Tactical 50rd drum mag for a few weeks, and I've been impressed with its reliability. It is not designed to lock the slide back, but otherwise, I have not had a single malfunction with the mag in 500 rounds of use.
That said, I still prefer standard Glock magazines for SD/HD, though this is a fun way to burn ammo at the range.
Believe it or not, you can still find these in stock and ready to ship for $130. I list two sources in the video description.