Hey Ed,
I've had a 12K for a number of years now. Admittedly, I think I bought it more as a novelty based on my time playing so many FPS games as a teenager, where shotguns were always among my favorite. About 5 or 6 years ago, I happened across one at a decent price and had to snatch it up.
As a whole, I very much like the gun. Its simple, dependable, and just plain cool-looking. I've never had any problems with it, but then again, I haven't exactly used it for all that much other than blasting paper targets.
I've never had any problem with ammo in the magazines, but I don't keep the gun handy for home-defense, so I tend to not leave the magazines loaded. I could see the soft-plastic of a shell being deformed over time, but would find it a bit unlikely. I guess one could just load one-fewer rounds in the mags to lessent the tension on the spring and keep the presser down a bit.
Two things I don't like about the gun:
1. The overall length. With the AK style reciever, this gun is long. I have but an 18" barrel on mine and it is among the tallest guns in my safe, making it a bit tricky to get into and out of my particular safe. For the last 3 or 4 years, I have been half-planning one of the sporterized conversions typical of Saiga's to cut down and make the gun a bit more manueverable. This would be moving the trigger forward, adding a pistol grip and a folding or telescoping stock. Unfortunately, the parts for this runs a good bit of money, money which I just can't justify on a gun I don't use.
2. Magazine availability. My 12k came with 1 5-round mag. Trying to find other magazines has been a bit problematic, with any that I have found being very expensive. Lately, I have seen a few more mags at recent gun shows, but costs are still usually $80+ for an 8 rounder. The .410 magazines seem to have been everywhere at the last show I went to, but still few 12g mags. I just can't justify spending that much per mag.
All in all, its a nice gun, but really doesn't add much other than a wow-factor to my collection. I have another auto-loader I use for hunting, an O/U for clays, a few other shotguns for this and that. With only 1 magazine, I don't consider this a good home-def gun, so I'd opt for my 1897 trench shotgun (which holds more ammo and is readily loadable, is far more intimidating, and heavy to serve as a good truncheon.) If I could get the parts for the conversion, as well as some extra mags, at a decent price, I would certainly do both. I really like the idea of a folding-stock, tactical shotgun with a slew of detachable magazines, but that would (atleast) double the cost of the gun for me.
Depending on what you expect of it, it could be a great addition to your collection. For me, the practicality is limited compared to some other pieces in my stock. I've actually considered selling it, but I just really like the looks of it, even though it rarely comes out of my safe.