The Steel Series Xai is one of the best gaming mouses out there, mainly for first person shooters. It has two extra buttons on each side which come in handy for things like reloading or knifing someone. It has a screen on the underside of it for quick on the fly setting tweaks. It also has a button right behind the mouse wheel that changes you CPI (sensitivity baiscally) with just one click. The feel isn't very ergonomic but that makes it perfect for FPS games because you don't play them using your wrist and only your wrist to move the mouse, you use your fingers a lot.
The driver software is extremely customizable to the point where it detects movements you dont even notice yourself. Lots of mouses have a straight line assistance. Whats that may you ask? its when you try and draw a straight line with you mouse either vertically or horizontally. You can change how much assistance you get with the Xai's software. You can make it damn near impossible with no assistance or way too easy. Why is this important? In a first person shooter, you want to have no restriction of movement while aiming and this helps like no other. Definitely check out this mouse!