Well, I just recently went all out and got the Razer Copperhead. I was going to go with something cheaper, and my friend convinced me otherwise. The way he put it: a mouse for your computer is like tires for your car.
Here's where a high-DPI mouse comes into play in games: If you're like me, you want to be able to turn around quickly without having to pick your mouse up off of the pad. Problem is, when you need precision (ie: trying to shoot that guy who's just a couple pixels tall), you can't aim for crap.
You'll be to the left of him:
+*
then move your mouse slightly and be to the right of him...
*+
(* = person, + = crosshair)
Never can quite get it on him... then you're dead.
The other option is to turn your sensitivity down, but then you end up having to pick your mouse up all the time to recenter it, and you have to make large movements with your hand, which can confuse the mouse if you do it too quickly.
So, you get a nice, high-precision mouse, and then you can set your sensitivity down to have that precision, yet still be able to spin around with the flick of the wrist. It's nice. If you're going to go all out and get an expensive gaming mouse, you should get a good pad to go with it -- one with a slick surface designed for optical mice.
Oh, the other thing high-precision mice let you do is disable acceleration in windows. This takes some getting used to, but in the end, you'll find being able to turn the same number of degrees consistently, regardless of what your framerate is or how fast you move the mouse, helps a lot in games.
Well, I've mostly just rambled. I just got the mouse, so I can't say too much about it, but I'm going to a big lan party this weekend, so we'll see how it fares. The more I think about it, though, the more I realize that a mouse is just as critical as any other component when it comes to gaming.
No, it won't magically make you a better gamer, just like getting a beefy new machine won't magically improve your skills, but it can give you enough of an edge to beat somebody at the same skill level as you.
Oh, I'll just throw out a couple more things about the copperhead I like: The settings are stored in the mouse itself, not in the drivers. You can set the buttons to be anything, even short macros. For example, I bound a macro that types "Hello" to one of the buttons, then unplugged the mouse and plugged it into another machine that had never seen the mouse nor did it have the drivers installed and I pressed the button and it typed "Hello." As far as I can tell, once you get the settings the way you like, there's no need to have the drivers even installed.