Well, I think that the Model 15 is one of Smith and Wesson's finest revolvers. It is a superb "Service Revolver" that will serve it's owner in any way one can be served with the ubiquitous 38 special cartridge. The 15-3 is the third variation of the model line that was introduced in 1949. It probably is the one most produced over the life of the gun and will handle any 38 spl. round very well.
$500 is ok if the gun is in exceptional condition or if it has accessories with it. It could be a little high otherwise, although, the gun is climbing in price/value as more folks collect them and find that they are one of the best 38 revolvers out there. I really like mine and would part with many other guns before I would part with my model 15. As I said, if the gun is in very good condition or has some nice accessories, it might be worth the five bills that is being asked for it.