I think the biggest selling point of the Serious Sam games is the engine they run on. The Serious Engine was remarkable for the time, and actually, still is. Instead of the relatively enclosed nature of some other games that came out at the time, Sam offered HUGE environments with dozens of relatively detailed enemies on the screen.
In fact, if you keep in mind the release date for this game, it really did have some pretty remarkable graphics, particularly in the gore area. Blood splatters everywhere are you kill hundreds upon hundreds or monsters. You also have some pretty cool lighting effects; pretty fancy for 2001.
The gameplay is standard, but not bad. You don't need to reload your weapons (a la Doom), and you work under the "if it moves, shoot it" mentality (a la quake). The game has around the same tone as Duke Nukem 3D.
Is this game worth it now? I don't know exactly. I like it, but the HD remakes, which have even better graphics and require only a slightly more powerful computer, are available on available on Steam for only 15 dollars, which I think I'd recommend over this version.
That being said, Serious Sam is not a bad game at all, and it would make a great addition to your collection.