Alright, i have to update my review since Croteam was really fast with updates. My first review was about the bad performance and crashes. But this is actually resolved now (at least for me), so here is an updated reveiw:

First of all, it's great how fast Croteam acted with patching all issues which caused my game to crash or render it unplayable. Good i didn't refund it.

The actual gameplay is exactly what someone would expect from a Serious Sam game, hundreds of enemies running towards you trying to murder you as fast as possible, while you are throwing tons of ammo at them. Miniguns, Rocket launchers, Snipers, Shotguns and plenty more weapons. Also you can dual wield them. All of them. Even the knife.
The humor of the characters are on point and a lot of references.
And as all Serious Sam games this one also offers a tons of secrets.

Now after all the issues are fixed and the game is actually working, it's worth it's money. :)