This is massively good fun. I don't think I've played anything similar since the Megadrive (when it certainly wasn't called a twin stick shooter, mainly cause we didn't have sticks).
Essentially, it's a top-down shooter, in which you must guide Sam through waves of seriously (see what I did there? Laugh a minute, me) persistent enemies who come at you in seemingly endless waves, using either brute force, guns or some sort of weird explody thing. Luckily, the exploding chaps tends to scream their little pixelated faces off as they charge towards you, which is helpful for spotting them.
You get the usual range of guns, which are pleasingly varied enough that you'll enjoy playing with them all (though my favourite is easily the cannon) and the enemies have neat little designs. I mean, it doesn't really matter. You'll be running around like a madman too busy painting the walls and floor with their blood and viscera to notice what they actually look like, but it's nice they put the thought in. It is hilariously bloody actually. Everything basically erupts into gushes of blood as you slaughter it, with the occasional eye left over. Fun for all the family.
Since it's Serious Sam, there's the usual secrets. I'm rather terrible at these, so have only managed about two. It involves a lot of shooting the walls. I'm sure you'll have fun finding them.