Well, since everyone else is expressing an opinion, here's mine-
I shot one belonging to a friend last year. Seems like a "fun" gun to play with, and no doubt it would be effective with buck shot loads at close range in a defensive situation.
Problem is, while most defensive shootings take place up close, not all do. So now you have several issues. Reduced stopping power and penetration is one. Another is lack of pin point accuracy - if several buck shot or pellets miss your target, you are responsible for where they end up, and that could be a consideration depending on where you are during a confrontation.
Round capacity, and bulky size are other issues that I think disqualify it from being a good choice for all around self defence. A modern hi cap auto, loaded with quality hollow points in a suitable caliber will be smaller, more accurate, and provide more reliable stopping power with increased cartridge count, an important consideration as not all felons come alone.
That said, sure it can be an effective weapon, I just feel its effectiveness comes in too narrow a window for an all around defensive weapon.