I love Painkiller; it's probably my favorite PC game of this year (and yes, I've played Doom 3). I didn't even particularly like Serious Sam (or than run-n-gun genre) but I love to sit down every week or so for a level or two of ass-kicking, physics-bending mayhem with Painkiller. I honestly think the physics are a huge part of why I enjoy it so much. Whereas endlessly shooting at enemies gets boring in other games, pinning them to the wall with a wooden stake or blasting them back 50 feet with the shotgun is always entertaining and different.