Trying to play some PC games can become an exercise in finger contortion (we're looking at you MMOs). Not only do you tend to have the entire row of number keys bound to abilities, but often a second or third row of buttons that need to be hit just at the right moment. Trying to spread the button jabbing load across both hands has given birth to a specific class of gaming mice that try to put as many buttons as possible at your thumb-tip.
Most notable of these is Razer's Naga, with its grid of twelve buttons on the left side. While it wins points for total hittable buttons, it is an ergonomic nightmare, and even after a few months of training oneself, it still feels unnatural to try and work the tiny numpad with one's thumb.
As you can see from the image, Corsair has placed nine buttons around the spot where your thumb naturally rests. Some of these are hittable with virtually no movement and are perfect for interrupts, taunts and other 'oh crap' abilities used in MMOs. Others are still easily accessible with a move of the thumb and all of their positions are easier to remember than the grid seen on the Naga.
While the thumb buttons are the most important part of the M90, the rest of the mouse is also a joy to use. Left and right buttons are suitably clicky and responsive, and the scroll wheel allows for both fine movement and large spins. The sensor is precise, and the metal and plastic construction both feels sturdy and looks great.
Annoyingly, basic functions are easy to program, but more complex macros can be hit and miss. This is most noticeable in games like SW:TOR, which lack ingame software macros, but for most people the functionality is more than sufficient as it currently stands.
As long as you don't need such elaborate hardware macroing the M90 is an absolute joy. Ergonomically sound, responsive and good looking to boot, it is our new favourite mouse for extended sessions of MMO gaming.