Awww man. C'mon. TRY IT!
All you need is the disk, install it, then download skulltag, then go to the Doom2 or whatever folder, and put the .WAD file into the skulltag folder, and...TA DA!
It has a mode that's kinda like Gears 2's horde mode, which is awesome. I guess the reason why so many people play it, is because it's easy to make custom content. New stuff comes out everyday. New maps, campaigns, everything.
Pretty frikin' awesome.