There isn't much that you actually need to know about Serious Sam..You shoot stuff and body bits go everywhere.  

If you insist, there is actually a story line. The game is set in 22nd century Egypt during an alien invasion. Humanity is losing the fight against hoards of alien soldiers. Sam is dispatched to do recon and ultimately use an ancient device (the time lock) to send a single person back in time to a pivotal point and alter events to save the human race...so it's actually a prequel to the other games in the series.  

A side note before I talk about what I like or didn't like...Never judge a game by its title. There is a setting in Not So Serious Sam that replaces enemy blood with confetti. (And that is all I am going to say about that!) 

Pros 

If you want to recapture that feeling you had when playing classic FPS's of days gone by, but don't fancy or enjoy the aesthetic then Serious Sam 3 is for you! What you get with this game is seemingly endless waves of bad guys that explode in a bloody mess if you plant a bullet in their face. Of course, you can add variety to your splatter spree...maybe a sledge hammer is more to your taste? 

I also love the advice on how to play.  
1.) Hold down the trigger/mouse button(s)  
2.) Run Backwards  
3.) Repeat  

Something that does bring the game up to modern standards is the detail, the textures, the models and descriptions are all bang on and shouldn't disappoint anyone that stops to look.  

Cons 

...If you could call this a negative point, modern FPS games have made me go soft (probably you as well). This game will kick your butt all day long in one way or another. You will possibly spend more time looking at the death screen than actually playing (I am presuming that you would be on a difficulty setting that is worth bragging about...I don't like tourists)  

This "problem" however will pass as soon as you get back into the groove! 

Overall  

It's pretty simple, if you liked the previous games...you'll love it. If you like shooting lots of aliens and painting the walls/floor/yourself red in bad guy blood, welcome aboard! This game has your name on it! 

...It's madness, and I love it.