I haven't heard Down, but Pantera is abysmal, so by the transitive property of suckiness, Down must be pretty bad as well. I won't go out of my way to find any of this stuff.
Nah, they're like completely different bands, just with the same vocalist. And Anselmo's vocals are far less irritating in Down than they are in Pantera.
If you have any interest in Southern or sludge metal whatsoever, I'd say Down is a must.