It's not a bad gun, but not as awesome for defense as you'd think. The receiver is LOOONG, so the thing itself is unwieldy in tight confines. While quality control is, well, Russian, the good news is that if the gas ports are done reasonably correctly (at least 3), you shouldn't have any problems. Vodka specials (2 or fewer gas ports)) are far from uncommon, so don't buy sight unseen; you want to pull the gas valve and have a look on the specific one you're looking at.
Conversion to a standard AK-like pistol grip and stock requires moving the fire control group forward, and that's a piece of cake.
You will need to make sure you break it in with heavy loads; these guns don't like being babied with light loads. Mine still doesn't feed all that well; every now and then a shell will nose-up into the top of the chamber, jamming up the works. It just isn't as reliable a feeding mechanism as you'd find in a more traditional tube fed semiauto.
Mag changes aren't as quick and easy as you'd think. Rather cumbersome, in fact. A magwell can fix this, but that's something else to buy and install.
Basically, the reason to use a S12 for defense is to be using a S12 for defense. If you want a quality defensive semiauto, then there's other options out there that will work better for less money with a greater chance of working right out of the box.