You must play this game.
I'm going to try to give you an "idea" of what this game is about, but it's not going to be easy if you know nothing of it. Basically, your an assassin. One. With 7 personalities (the Killer 7). Each of your personalities has a unique ability that will help you solve puzzles throughout the game. Your opponents are the Heaven Smile. A group of some pretty crazy things, but I don't want to spoil any story so as vague as that is, I'm going to leave it at that.
The gameplay of this game seemed to get criticized a lot in all the reviews I read when it came out. Personally, I think it was done awesome. The whole game is set on "rails." This means that your movement is slightly limited, but the levels are so confusing and the paths wind in so many directions that if you weren't on rails I guarantee you would wander aimlessly for hours upon hours. The way you move is you press one button to run forward (A on the gamecube) and on button to turn 180 degrees (B on the cube) so you can go back. When you get to forks or doorways you choose which way you want to go with the control stick and select it with A.
If it doesn't sound fun, don't even worry, it works out fine, I cannot stress this enough.
As you run through levels you'll of course come across the Heaven Smile. However, they are "invisible" and you'll only know they are there because of their eerie laughter. When you hear a laugh thats your cue to jump into first person view (hold R). In here you can scan (with L) and when you scan in the direction of an enemy they will become visible. Each enemy has a weak spot (some take more effort to find than others) which is labled with a nice gold area. If you shoot the gold area the enemy will explode into a splatter of blood with just that one shot and you will collect the blood which you can later use to level up. However, theres also the choice to just load the enemies full of bullets which will kill them, but you will not receive any blood.
This is where one of the minor flaws in the game comes into play. You'll find that for certain enemies that have weak spots on their upper thighs you can't always see them because their arm will get in the way. Other times it will seem the weak spot isn't there at all. Lastly, are those times when you swear your shooting the weakspot but you just blow off the leg or arm and then the weak spot is gone for good. These would be more of a concern if you barely collected any blood throughout levels and you needed every little bit, but there are enough enemies to make up for the few times you'll encounter this little problem.
Hearing the name Capcom alone should alert you that there are many little puzzles in this game that you'll need to solve to progress. Some are very obvious and some aren't. Some of the not so obvious ones can leave you feeling very frustrated. There's no way around it. You may even solve it and still have no clue what they were intending you to do. Some bosses require certain strategies to complete as well (...fix your tie...) that may leave you wondering. I haven't really much to say about this, just making you aware.
The games story is crazy. It twists, it turns, it doesn't make sense, it does make sense. I'm not going to make sense of it for you. I say, just take it however you take it and leave it at that. Play the game a few times to try to understand more. I think it's awesome because of the confusion, other people don't like it so much. I see no reason why a little confusion should stop someone from playing this game. Plus, there's some really awesome stuff that goes down towards the end.
I should probably stop talking now and just give my final thoughts. This game has a few flaws and is definitly not perfect, but....it is amazing. One of my favorite games for sure and one that I feel everyone should play through at least once. Put your previous thoughts aside and pick up a copy of this game.