I find that the xai mouse is the most comfortable and effective for use in FPS games. It's stylish but effective. I like it because it feels a lot like an upgraded version of the default windows mouse, which I had used growing up. Tricky thing is, they are hard to find so you're probably best having a look for one online, or using Gaming Gear Shop (.COM) when it's launched if you want to buy.
The xia has a lot going for it. Not only is it comfortable, you won't find annoying buttons anywhere that you won't use and are just nonsense. The xia was built for precision gaming and it's great to be able to have all those settings to play with to get the mouse feeling the way you want in game. A strong cable means that the xia has the potential to last many years in service.
It's a simple yet elegant design, which is purpose built to deliver precision and accuracy every time. I strongly recommend buying a xia as soon as you can. Watch your play improve over night, after you have gotten used to it, you'll see that it is the most easy to point and use, whilst being one of the most precise and economical mouse out there. If you can find one. ;)