Painkiller was something extraordinary for me, though some people will say that it just like ut2004 grab your gun and rock and boom all them monsters.
Well it's not just that, you have some minor goals you can achieve to unlock special throat cards that gives you ultimate abilities like Haste, taking half of damage, Invulnerable and more... the movies between each 1 Main level(each main level divided to 4-5 sub levels to complete the experience of the game at the different locations ) aren't the best but they emphasize the story well.
the gameplay defiantly 10, it can be boring at the first 2-5 minutes but once you get into it you won't leave it, you shut gun will help you blast your way through the game, the environments changes very frequently unlike Doom III, Painkiller designers did great job on the locations, they are very varied and gives you feeling that you really in the game...
to sum it all up the game worth buying, totally one of the best games that you can ever play... HIGHLY RECOMMENDED.