Everyone has an opinion and it is a YMMV thing but I cannot even think of a reason to own a .500 S&W except and unless you use it as a hunting gun for really big game or just because it has a cool factor. I owned a .416 Rigby for both of those reasons until I tore my rotator cuff. Didn't keep the .416 after that.

For self defense, the .22 beats the .500 every time. Faster to get into play, easier to shoot, easy to punch a bunch of small holes in the perpetrator without missing, etc.

The sole reason a .500 is useful for self defense is because no matter what happens after the first round goes off everyone will be shock, anyway.