The sheer intensity of this game covers all its flaws.
In Second Encounter, you travel through 3 different eras in time, collecting relics that will allow you, hopefully, to reach this alien overlord who is sending his minions to Earth. The story doesn't offer a great depth, and it is never really explained exactly how finding the Holy Grail takes you to space. Regardless, the game still succeeds. Why?
Because Serious Sam isn't about the story. It's about the GODDAMN INSANE shooting of a CRAPLOAD of aliens with an equal (or greater) MASSIVE amount OF BULLETS, LASERS, MISSILES, etc, etc, etc.
Serious Sam definitely succeeds in capturing your full attention as you deal with different enemies in various environments. While there are not that many different minions, there are enough to confuse you with which weapon to use against who (You don't wanna waste a missile on a petty skeleton, or attack a robot t-rex with a handgun). You will be quickly immersed in the outdated, yet sufficient, graphics because of the sheer intensity of the gameplay. The game is not particularly long, but the length is apparently just short enough to avoid repetitions.
A game worth experiencing at least once. Even for a non-FPS player like me.