If you're serious about your PC gaming, you've already heard of SteelSeries. You might remember their first high-end mousepad (excuse me, "mousing surface"), their , possibly even their highly-acclaimed Xai gaming mouse. Lately, they've been going hard with licensed gear like their , but with the Sensei, they're going back to their professional gaming roots. This mouse is intended to replace the Xai, which means it's only for the best of the best. (Of course, the $90 price tag already told you that.)
The processor is used to let you tweak every single mouse setting you could possibly ask for. The major feature here is your mouse sensitivity; you can tweak the mouse's actual hardware counts-per-inch sensitivity from 1 to 5,700, but if you want to go higher than that (maybe you work with multiple high-resolution displays), the processor and built-in software can "double" your CPI up to 11,400 CPI.
You can also control the lift distance, mouse acceleration/deceleration, and set the drivers up to fine-tune (or eliminate completely) any mouse path correction functions. What's more, you can load your profiles onto the mouse itself, so if you want to bring your mouse to a gaming event, it'll keep your exact settings without having to install any special driver software. There's a small black-and-white LCD display on the bottom of the mouse that you can use to change mouse profiles on-the-fly, and it'll show your image of choice when it's idle.
Like any good gaming mouse, you can configure different profiles for different games and applications with the included SteelSeries Engine driver software, which lets you change your 7 button assignments. You can even develop some pretty in-depth macros that track timing and include keystrokes, though those probably won't be tournament-legal for your game of choice. You can also track your mouse usage sessions to see which buttons you use more often than others, and change the colors and brightness of the built-in lights. You probably won't actually do much of that stuff, but it's kind of cool to know you can if you want to.
The mouse itself feels comfortably large, close to the feel of a classic Microsoft Intellimouse. The side buttons are a little bit too easy for me to hit with my standard grip, meaning that I get a few accidental back/forward commands while I'm browsing the Web, but it hasn't been a problem when I'm doing something more focused like playing Starcraft 2. The surface of the mouse is a rather attractive metal-coated plastic which looks good and feels a bit lighter than my Razer Naga, and the mouse buttons themselves are responsive and give a satisfying click.