Painkiller is truly one of those games that pays homage to the classic FPS, defining a very simple experience, with a few tagged on bits, a rather fractured, but otherwise viable story, and hordes of beings unsavory with which to test a cabal of death machines upon. This game truly brought classic FPS into the 21st century, and then sprayed it's body parts all over the place in grizzly fashion like any respectable FPS would.
The descent into Hell is no easy task, either, as you make you way through truly unique and colorful levels, falling further into the depths, through plague-stricken town, old and forgotten monasteries, cavernous tombs, military bases, and even an old opera house; all while fighting off an almost Serious-Sam-esque number of enemies, as they throw everything from bullets to grenades to spears to internal organs at you to stop you from reaching your goal of putting Lucifer back in his place. Indeed, it seems the road to redemption for our hero is a long, bloody campaign into the abyss, and with a splendid venue of visual effects and stunning use of the havoc physics middle-ware, it's certainly a fun ride straight to Hell.
If you love the old school games, or are just looking for an alternative to newer realistic shooters, centered on militaristic combat rather than rampant mayhem, you'll wanna own this ever so delightful romp through a vast multitude of levels and enemies..
Oh yeah, I forgot to mention that the bosses are usually in the 3-to-15 story tall category. ;) Good luck with that