On a technical level, the game is terrible. It doesn't offer Doom 3 graphics. It doesn't come close to having such great traditional deathmatch. But that's deceptive.
First of all, the graphics are absolutely ridiculous for such a game. It renders the biggest environments you will ever find in any shooter. And it does this so well that the game runs excellently on my antique Pentium II 500MHz computer with nearly no lag on multiplayer (which I'll get to later).
Second of all, the level design is beautifully scripted. There's one spot in the game where upon picking up a small health pack, the game drops another ahead of you. You pick that up, another spawns ahead. You keep on going, and all of a sudden, when you pick up one of them, a HUGE flood of monsters spawns right next to you and as you flee for dear life you'll most certainly see what it's all about.
Another level has the floor falling out from under you and having you land on a giant trampoline-like floor. As you uncontrollably bounce up and down, trying to recover your senses, a massive wave of kamikazes charges in, and you'll definitely be screaming "THIS IS INSANE" as you attempt to fend them off, all while both you and your enemies are flying in the air, and as you hear the mindless screams of the enemies pitching higher and lower thanks to the Doppler effect.
One of the final levels has you jumping through a portal and appearing on this absolutely massive plain of just sheer ice. As you gaze around and admire the stunning beauty of it, in the very far distance you hear the screams of enemies, and the next thing you know, you'll be fending off huge herds of werebulls, giant mechanized robots, demons, and of course kamikazes, all of which are sliding around all over the ice. That level in particular is literally huge, and it would take at least 10 minutes for you to cross from one end to the other. You really get the feeling that the developers simply LOVED making the game, and really playtested it thoroughly to make every bit of the game pure sensory overload and aboslutely insane fun.
Third, the audio in the game is excellent. You'll never, EVER, have the same feeling as you will when you enter a giant meadow, seeing only plenty of ammo in front of you, and hearing the music starting to throb as you realize what a massive battle you'll have ahead of you.
Fourth, one I've alluded to many times by now, the battles are something you can't forget. Nothing quite matches the feeling of seeing an absolute swarm of giant chainsaw-wielding people with pumpkins for heads charging at you. No other game throws this many enemies at you. Ever. As GameSpot says, "Some battles seem to stretch on indefinitely, never letting up, only escalating and never giving you the opportunity to conveniently reach over to quick-save. Even if you do know what's in store around the next corner, defeating wave after wave of monsters is never an easy task, as your reflexes, your precision, and your endurance will constantly be put to the test…Time seems to stand still during some of the more hectic battles in The Second Encounter--it'll seem like you're fighting an endless onslaught of foes, burning up all your ammo for all your weapons in the process. Suddenly it's over, and you'll hear yourself exhale as though you were holding your breath for all those minutes. The action in Serious Sam: The Second Encounter is some of the best you'll find in any shooter to date. Calling it mindless is selling it way too short." It might seem repetitive, but it is never, ever dull.
Fifth, as you should have guessed by now, it's so damn fun. There's a genuine sense of humor in the whole thing. Should you feel queasy at the sight of blood, for example you can turn it into "hippie" blood, making monsters bleed flowers and fruit and "happy stuff" instead of red blood. In addition, Sam cracks some hilarious one-liners throughout the game. "I hate running backwards," he complains on more than one occasion as you're desperately trying to fend off a tidal wave of monsters. Should you die, no problem. No running back for 5 minutes back to the battle. No loss of weapons. No loss of experience. No waiting for the spawn time. You just pop right back in and continue breathlessly fighting. It seems like a bad idea, and it would be in any other game, but Serious Sam is a game so tough you can't beat it without dying at least a hundred times. I died in my game nearly 300 times.
Now, all of this makes Serious Sam a good game. It's entertaining, but probably not something that makes me love it so much. There's one more gigantic factor: Multiplayer Co-op.
That's right. This is one of the very very few PC games left which allows you to run through the game with friends (or strangers) at your side. As the number of people increase, so does the monsters, and having an 8-way LAN party trying to crash through the game....well, there's nothing quite like it. Trust me on this. Even online, after playing just a couple levels with some strangers, you'll feel as if you knew them your whole life. You can even play what they call "Seriously Warped Deathmatch", a unique twist on deathmatch. It actually rewards you for picking up as few items as possible, so you're constantly pressed: do I pick up this health and let my future killer get some more points, or do I really need it now?
Oh yeah, it's a budget game. Under 20 dollars. Almost a third of the price of Doom 3, which doesn't offer co-op, which requires a powerhouse rig to run, and which isn't just that much fun.
In short, Serious Sam is quite possibly the best shooter ever made. Others always have small detractions: "This game's single player isn't very engaging" "The graphics slow your game down" "It's beautiful, but not much fun" Serious Sam has none of those. As GameSpot said when it gave Serious Sam 1 its Game of the Year award in 2001 (over Max Payne), "More than any other PC game developer this year, Croteam seemed more acutely aware of what PC games are all about in designing Serious Sam--they're about fun. And for your money (or for twice your money), you'd be hard-pressed to find more fun in a single PC game than you could in Serious Sam." I think that sums it up well. You can find other games that offer much better graphics, much better sound, but you simply can't find another PC game that is as fun as Serious Sam, The Second Encounter.