Personally, I look at the SPAS-12 like a Delorian.
Does it look cool? Hell yeah.
Does it perform as well as it looks? Eh...
Everything a SPAS-12 does, a cheaper shotgun can do better. It's a cool novelty, because it has such a unique and awesome look. From what I've heard replacement parts are very expensive (and perhaps difficult to get ahold of), and the safety isn't really terribly safe and misuse can damage the gun or in some cases cause it to fire. Add to that the fact that it's quite heavy. They aren't a BAD weapon, but they aren't really a good one either.
Would I keep one if you gave it to me? Yeah! Give me a Delorian too...I'll probably never drive it but I'll probably never shoot the SPAS-12 either.
-Jonathan