David Bjur said:
OK, thanks for the answer! Haven't played most of the old school FPS's because I'm more focused on single player than multiplayer.
Then I'd suggest to pick up Serious Sam 3 when you see it on sale or such. It's freakin hard (play it on Serious, in the last 10h I got through half the game [and this isn't even the hardest difficulty]), the humour works (unlike DNF it isn't played straight and the lines fit the situation) and it combines some modern mechanics with old and creates a fitting expirience for modern times (you have sprint and iron sights with two weapons [which just increase the accuracy on LOOOONG range] but no regenerative HP).
A pro-tipp: A boss in Serious Sam 3 is just the introduction of the enemy you'll get in dozens during the next levels.
It's really funny to say that it is one of the most intelligent shooters I played in the last few years. In the last segment I was stuck in a narrow corridor that lead to a big open place but upon entering the coprridor enemies started spawning. I had to win ground first so I could fall back when needed (all enemies hit like trucks, some of them are fast as hell and can take a load of punishment so I need to have room behind me) while spreading C4 on the ground as a "Oh Shit, too many Kleers" button. While doing that you have to dodge projectiles and pick up the really deadly targets first (in this situation: Kleers. They are fast as hell, can kill me in 4 hits while I am full on health and armor and always come with a load of buddies). The amount of prioritizing and strategical thinking during this game is quite astonishing if you see that it is presented as a 90s esque testosterone FPS.
Also, there is (hold your seat) split-screen multiplayer which includes up to 16 player coop campaign (16x more enemies). A PC exclusive with split-screen MP!

(The PVP MP sucks ass. Bad weapon balance, bad spawnpoints, bad mapdesign...)
If you like FPS (especially hard FPS) it might be worth full price, if it would be your first expirience with oldschool FPS I'd suggest to keep your eyes open for sales. For the 20? I paid for it it's a blast.