Resident Evil 0 is a game unable to stand on its own two feet- an installment unworthy of the Resident Evil title.
Resident Evil 0 is a game that can't hold up on its own; the plot relies heavily on your understanding of what happens in the original game, and even if you have that understanding it still falls short of what you'd expect from a Resident Evil title. You control two characters, Rebecca Chamers (a member of S.T.A.R.S. Bravo Team and an accomplished Chemist) and Billy Coenn (A prisoner on his way to being executed), and swap your control over them to solve puzzles and other mysteries through-out the game.
From the beginning, the game seems... different. The title screen tries very hard to be scary and fails and the opening narrative requires you to know quite a bit about Resident Evil for it to make any sense at all. On top of that, it goes on about "uncovering the truth" about Umbrella and mentions Sheena and Rockfort Island, yet never mentions anything about those islands and what they have to do with this game anytime afterwards.
Now then, to the gameplay. It plays just like any other older Resident Evil game; the camera is in a fixed position, your character's movement is awkward but manageable, and you rely on a gun with a limited ammount of ammunition to stop all the bad guys. Like I said earlier, you swap between the two characters in the game whenever you want, but are forced to in order to solve several puzzles in certain parts of the game. Although the idea seems neat, it gets very old and annoying quickly. Through-out the game there are many puzzles, normally harder than they need to be and un-exciting (finding a clock to put a gear in?), which does little to help the game be fun at all.
There are several new enemies that make an appearance, most notably the Exterminators and Leeches. Exterminators are actually decent monsters; smaller white monkeys with a ferocious appetite and a bad anger problem. Leeches are... well... Leeches. They serve as the main "enemy" in the game, and many people in it are incredibly afraid of them. As for bosses, they're one of the more redeeming parts of the game. Giant Bats, the Tyrant, and other assorted animals, insects, and bioweapons are mixed in to keep you wondering what you'll be afraid of next.
The story, on the other hand, needs improvement. Like I'd said before, it requires a lot of knowledge of Resident Evil games to understand (which may make it good to some people, but to me it makes it incredibly annoying). You start off searching for the ex-prisoner Billy Coenn in a train, which mysteriously starts to move and crashes into an Umbrella Management Training Facility that seems like an attempt to mimic the mansion in the first game and, like the mansion, leads to an Umbrella Laboratory. Not far into the game at all a mysterious character emerges who says he's responsible for all the happenings in the first game, and then goes on to try and kill you and everyone else in the area. You end up having to stop him and uncover the mysteries in the lab surrounding him and what all Umbrella has been up to. The game tends to be not nearly as hard as other games in the series (It only took me a few hours to beat it, unlike all the others), although it does have a nice mini-game afterwards where you have to collect green and blue leeches through-out the training facility. The more you collect, the neater the rewards you earn in-game (unlimited ammunition for your handgun, special guns you can't find in the main story, etc.).
In the end, Resident Evil 0 leaves a lot to be desired. It can't stand on its own two feet without the support of Resident Evil 1, and even with that support and the support of all other games it still doesn't give you a memorable experience. This installment was mostly to let players understand why things happened in the first game, and tried making things fun and enjoyable in the mean while but ultimately failed. If you're a crazed fan of the series, go ahead and search for it in a bargain bin (although you could always just save yourself the money and read a plot summary). Newcomers to the series should beware though; this game will be more of a hassle than a thrill.