I bought a 9 round mag recently at a gun show. I wanted it for when I only carry the P3AT and need a spare mag. I used it for the first time at a pistol match on Sunday. I didn't put a ton of rounds through it, but it went without a hitch. I hope to put 50 or so through it plus some carry ammo before I declare it GOOD. It is weird with the grip being longer than the gun.
The only issues I've encountered with mine, is that there is a space between the handle of the gun and where the handle extension on the magazine begins. Just enough to pinch your finger, especially on recoil. This might not be the same for every mag, but that is what it is for mine.
Other than that, mine has worked without a hitch so far.
Plastic material that grips the metal part (this part is same length as regular mags) was thin enough to separate from the main housing. Basically this is a grip extension that holds extra rounds. Maybe they have improved their design since then. I was one of the first to order when they came out.