I'm a PC gamer for a few reasons:
1) It's games are always a generation ahead of consoles.
2) Hardware can yield you better graphics and smoother gameplay.
3) The PC is an open platform, there's tons of mod support for games (maximum replay value).
4) I prefer a gaming mouse and keyboard to a gamepad (where you sometimes have too few buttons and are limited to axes of motion).
5) My PC cost me less than each the Wii, the PS3 and the 360.
6) The PC does a lot more than games.
7) Games for PC usually have a higher standard.
8) PC hardware pushes the gaming market on all platforms.
9) (As Perfect7 said) A PC runs console games better than some consoles do.
10) Not everyone wants to pay a fee for online play.
11) Xfire and Steam alone are better than XBL.
12) Microsoft is NOT good at making hardware.
13) If something goes wrong with my PC (which rarely happened), I can fix it myself without voiding a warranty or waiting 4 weeks (some console users may find this a bad thing).
However, I own a Wii and it's pretty fun, and I wouldn't mind a PS3 for GT5 and Blu-ray playback.