It should be noted that the version of Serious Sam that I played is Serious Sam, The First Encounter Special Edition. This limited the game to 7 levels out of the 15 available on the full game. For some reason I was always hesitant to buy this game, but finding it in the bargain bin for $1.99 goes a long way to overcoming any hesitation and 7 levels is enough to form an opinion.
The style of this game reminded me very much of Quake. The controls are very easy, simple and fast. Health, ammo and weapons are lying around and picked up by simply moving over them (a feature I always appreciate.) There are also secret areas for additional weapons and power-ups. There's a secret level in this edition called Moon Mountains so be sure not to miss it. There is also another secret level later call Sacred Yards.
The story, although slightly more fleshed out in Serious Sam, has some similarities to Quake. The year is 2104 and deadly monsters have teleported in from a different dimension and are attacking the colonized galaxy. Earth and humankind are now in danger of extinction and Serious Sam is sent back in time through the power of an ancient artifact called Time Lock. In order to change the events leading to this attack, he must first find some answers in the ancient cities of Memphis and Thebes.
With this Egyptian theme, the game environments have all the associated grandiose temples, buildings and landscapes. The graphical maps are very well done if somewhat sparse, but the 3D engine allows for some impressive effects including the great number of enemies that can be rendered simultaneously. I also understand that a HD version of the game can be purchased if you prefer this to the classic. There is the usual compliment of weapons common to shooters; knife, pistols, shotguns and rocket launchers as well as a laser gun. The occasional humorous comment from Sam when he is about to face dire trouble is very amusing. Sam has a small computer implanted in his skull that allows him to analyze new situations and it also keeps track of your statistics for each level.
I played this game on normal and it was quite difficult as it teleports enemies in and throws mobs of them at you. The sheer variety that you have to deal with at the same time makes it very challenging, especially since it's fond of locking doors behind you. One level locks you in a room with about 300 creatures and you must kill all of them to advance. I've never liked that kind of situation in a game where numerous uses of quicksave might be your only option as you run out of health and ammo to boot. Most of the enemies in this game move faster than Sam, which is all right if you can keep your distance and kill them before they get too close. In a locked room this is near to impossible. Fortunately, this tactic was only used once, at least in the Limited Edition.
I'm a little undecided as to whether I'll buy the full game, not because it's not very good for its type, but it's basically one level after another of doing the same thing. There's no denying that this game is popular and received high reviews, but you should know what type of game this is before purchasing it. If one killing spree after another is your thing then Serious Sam is probably one of the best and most challenging around. This game can also be modded for those who are interested in that aspect.
The original Gold Edition is a special release featuring The First Encounter, The Second Encounter and a fan-made level called Dark Island. Watch that you don't get confused with the later HD releases. There is some information for both the original and the HD versions in the links. The upcoming Serious Sam 3 BFE will be a prequel in time to this one.UPDATE Aug 2015: As the classic trailer is just so bad and not really representative of the game, I've included the HD trailer instead. Availability is mostly the HD version. The system specs are also for the HD version.