The most disappointing title in the Resident Evil series.
Resident Evil 0 begins as any other title in the series: opening cinema, then you meet the character you'll be playing. You're playing as Rebecca Chambers, a member of the S.T.A.R.S. Bravo Team a chemist. You start as Rebecca but later you team up with Billy Coenn, a prisoner that was being transported to his destination for execution. You'll realize soon that you'll need to swap characters often throughout the game to solve puzzles, get items, and sometimes to defeat enemies.
This might sound exciting but the co-op can sometimes become frustrating. Billy and Rebecca both have the same amount of item slots, and often you'll find yourself leaving items in various rooms so that you can pick up ammo or items that you need to progress in the game. This can be frustrating if you don't remember which room you left the items in, or if you left an important item in an area that you can no longer get to without traveling back a lot. In case you haven't noticed: There's no item box in this game, making it difficult to locate some items and to retrieve them. Another possible annoying aspect of the game is that you'll realize Rebecca can take VERY few hits before landing in the red "critical" zone, Billy essentially becomes the "tank" of the game. Now let's discuss enemies, story, and puzzles.
Enemies are the same as in other RE titles. You'll encounter zombies, Hunters, and two brand-new enemies: Super Leeches and Exterminators. Super Leeches are pretty self-explanatory: they appear somewhat like a person but it's just leeches that have teamed together to attack you. They aren't easy to defeat if you don't have the proper weapon/ammo and sometimes it's better just to dodge them. Exterminators are white monkeys who will jump at you and run at you at fast speeds -- they'll either hit you or bite you, and it's best to avoid them if you can. Bosses are probably the better enemies in the game. You fight a giant, a centipede, and even the Tyrant itself. The bosses are probably the funner, more "challenging" part of the game.
In terms of the puzzles they aren't terribly hard to solve. Some will require you split Rebecca and Billy up to solve, or to obtain certain items (such as the bit where you're in the lab areas). The puzzles can confuse you but once you figure them out you'll be asking yourself "Why didn't *I* think of that?" overall, not too bad on the puzzles.
Storyline wise you will most likely want to play a different Resident Evil before you play 0, because it requires that you know and understand some of the events that took place in other installments such as Resident Evil, Resident Evil 2, etc. You'll also be able to understand how the characters and events in RE0 are linked to the later events in the story. You'll be happy to know that you're able to learn the origins of the original T-Virus and the later G-Virus, as well as the original founders of Umbrella Corporation, and how things started to "go bad" within the company, as well as who caused the first "incident." You'll learn all of this but you're left wanting more somehow.
Overall the game is really only worth the knowledge you learn from beating the game and discovering it's secrets, not so much the gameplay, difficulty or enemies. Although the game spans across 2 GameCube discs, you'll realize that it is also very short and really won't take you a day to complete unless you're struggling with it for some reason. The only bonus you get from completing the game is the Leech Hunter minigame, which unlocks other bonuses for you in the main story.. But you may find you don't want to play through the game again afterwards.
The bottom line, once again, is that the game is really only worth the story that it holds. You're probably better off just searching up a plot summary of the game than you are searching for a physical copy to play and complete -- If you do it anyway, you'll realize quickly that this game may reveal the origins, but it is certainly not the best or the funnest game in the series.