The formula used in Serious Sam is so great that even an average rip-off from it looks good.
First of all, you play with an archeologist named Will Rock, who is trying to stop the Olympian Restoration Army from resurrecting the old Greek civilization. Their plan has already begun, and your character 'will rock' (that sounds very silly) while beating the hordes of monsters from the Greek mythology.
Before you think Will Rock is a bad game, let me say that the action sequences are usually good, not to mention the graphics engine that is really impressive. I found most levels very well designed, and so are the artistic paints on the walls, floors and objects. Some monsters are so well animated that will keep you interested for a while. There're satyrs, minotaurs, ancient warriors and even living statues. Unfortunately, the undeads don't look good, and that's particularly bad because you'll see them very often.
So, what's ripped from Serious Sam that's not right? To start, several weapons are useless, not because they're weak, but due to the fact you have at your disposal really overpowered ones. I used only five weapons in the entire game, despite the fact you have eleven in your arsenal. It would be spoiler if I put here all the weapons you'll get, but let me say that the 'bizarre' ones are the useless stuff. However, I think it won't hurt if I speak about the earlier weapons: The pistol has unlimited ammo, and fires with a great precision; the shotgun, although is bad designed, is extremely powerful, no matter what distance is your target; another bad looking weapon is the machine gun, but it's very useful because it fires with good rate and doesn't need to be reloaded (while most weapons do).
Another problem are your enemies. Most of them have the same behavior, and you deal exactly in the same way with all of them. Obviously all your foes must be killed by shooting, but you don't have actually a strategy to beat them. In Serious Sam (sorry for the constant comparison, but it is necessary), most monsters were killed in different ways. For example, the bull you have to sidestep when he attacks, and them fire against him from behind while he's maneuvering. There're many other examples, but in Will Rock it's always the same pattern: dodge and shoot, without any tactical decisions.
The levels aren't very different from each other, although they all look great: sky, buildings, mountains, water, pedestals, destructible objects etc. There are only 10 levels, but they're quite extensive and provide a good 7 or 8 hours of gameplay in total. There is some lava jumping, as well rooms with some lethal traps. Many of these moments will probably remind you Montezuma's Revenge, because if you miss (and, in general, that isn't a difficult task), you die. Depending on your taste, that can be a pain in the ***.
If you're expecting huge arenas, then you'll be disappointed. In Will Rock, there're some large areas, but nothing colossal. At least, the boss battles are awesome and provide a good challenge, especially in the final confrontation. I played in the regular difficult setting, and I had some trouble in several places, which provided really good, disputed moments.
I found some minor bugs even in the patched 1.2 version. In some of the boss battles, I could simply ignore the giant monster by easily opening a door and ending the level. The intention, obviously, was to let the door locked, to be unlocked only after defeating the boss. But I decided to go without cheating and killed the giant, and so the same door opened (as if it couldn't be done before). Also, I once got stuck between a wall and a big door that was just opened. So, I used an explosive and hurt myself to get way from that unbelievable prison. There're some few graphical problems, but they don't mess with the gameplay.
Sound effects and music are fine. The soundtrack isn't much variety, and sometimes the same music is repeated over the levels. But I didn't find it that bad, since some of the background music is rock or heavy-metal. Naturally, it's your taste that will say if it's good or not.
Will Rock deserves a credit by offering a simple, but working multiplayer. You have the 'Deathmach' and 'Cooperative' modes, which extends the life of this game. As I said, you can finish the game pretty quickly, but you can replay it in a higher difficult setting (called 'Will Die', very hard) with some friends. So, if you're an old-school gamer and liked Serious Sam, give Will Rock a try.