It depends. If you only want to spend as much about as a 360 costs, then don't bother. If you're willing to spend $700-800, then you can build a computer that trashes games. If you spend more than that, you get godlike power.
That being said, do graphics appeal to you? Because even an entry level gaming PC is going to be a hell of a lot more powerful, and make things look WAY better, than an xbox.
The other side of it is mouse and keyboard - most PC gamers wouldn't consider using an xbox controller to game, even though it can be done. You've got a LOT more freedom in what you do with a mouse and keyboard. (They're faster AND more precise)
The other noticeable thing is that competitive games like MW3, Battlefield, ect.... are a lot faster paced on the computer, due to lack of any aim assist and the nature of the beasts. Personally, I like this better, because it cuts down on camping and makes the game more fun. A lot of people dislike it, because it gives the game a sharper learning curve.