Cast me into the specific-holster camp.

However, I will add some exceptions for specific applications.

For a pocket holster, I prefer less retention. What I don't want is the holster coming out with the gun when I draw. So I typically use a Nemesis holster, which is not gun-specific.

I use Remora holsters for my larger guns when they're on nightstand duty. Again, I want less retention so I can draw the gun easily, or shake it off the gun if my draw technique is less-than-ideal. FYI, I use nightstand holsters to keep me from inadvertently sticking my finger in the trigger guard before I'm fully awake and aware.

I usually carry my 642-1 in a Wilderness Tactical Renegade ankle holster as a BUG. The holster body is made from heavy-duty elastic. While it is snub-shaped, it is not molded specifically for a J-frame. I'm sure any similarly-sized snub would fit. However, because of its elastic nature, it holds my revolver very securely without retention straps. While I haven't done any running with it, aside from maybe jogging across a crosswalk to make the light, I have done quite a bit of hiking, some of it on some difficult terrain. The gun has never shown any indication of coming out. I've also tried pulling the gun out of the holster when it wasn't strapped onto my leg, and I have to say it took a lot more effort than I thought it would.

But other than those specific applications, I think it's best for a holster to fit a specific gun. I should add, if I were using a leather ankle holster, I would definitely want it molded specifically for my gun.