This is definitely a worthy successor to the first few Serious Sam games with lots of crazy shooting action. Some new weapons have been added with different capabilites, which add new tactical features to the game.
For example if you are standing in a rather narrow corridor and know that in a few seconds hords of nasty enemies are going to pass through to pay you a visit, why not greet them with C4 :). It can be placed anywhere in the game and explodes when you push the button.
Fun guaranteed ;-)

Anyone who has played the first game will love this sequel. The further the storyline progresses the more insight you gain into the initially confusing plot. Penumbra Black Plague starts right off where Overture ended.

Regarding the level of fear and terror instilled on the player, this game has even managed to surpass it's predecessor as (different from the first game) you find no weapons like a hammer or a pickaxe to defend yourself anywhere in the game. Considering self defense your only options are either running away or using rocks, planks or any usable trash lying around.
The enemies are also a lot scarier ^^.
It is possible to use your environment to your advantage as you will find hiding places or rooms where you can (sometimes only temporarily) lock enemies inside.

I don't want to spoil anyone with the exact plot, but my conclusion is:
The Penumbra series is by far the best horror adventure game I have ever played - both gameplaywise and storywise.

Are you currently experiencing constipation?
Well, you might have just found the solution to your problem. Playing this game in a pitch dark room with your speaker volume cranked up near the maximum will make your digestive system work like a steam engine once again.
(An overdose of the product in question may lead to involuntary discharge of various body fluids)

Use at your own risk!