A post running underneath the wrist rest holds up to seven round 6-gram weights. To release the weights you unscrew a metal knob from the top of the post, but the knob itself is actually an Allen wrench that you use to make various adjustments to the mouse's grip. You can store unused weights in a small canister that plugs into the receiver/battery charger, and unless you use all of the weights, a small spring-loaded cap slides down on top of the post to keep the active weights in place.
With the post/Allen wrench, you can adjust both the longitudinal depth and the left-to-right cant of the thumb grip. Adjusting this piece is important, as it will determine your comfort with the three thumb buttons. Two of them sit in the familiar forward-back position for convenient Web navigation. These are the only two you can reprogram, however. A third button sits a bit farther forward on the thumb grip, and provides the unique service of temporarily adjusting your sensor dpi setting.
The idea behind this button is to let you briefly slow down the cursor, thus providing better control when you're temporarily zoomed in on a target in a first-person shooter. Hold the button down to snipe, let it go when you're back to more fast-paced shooting or running around. This is a sound idea, but we'll admit that in a few rounds of Modern Warfare 2 multiplayer, we found we had to force ourselves to remember to use it, at which point our target had moved. We expect it would become more comfortable with practice.
On the opposite, pinkie-finger-side of the mouse, Mad Catz lets you choose from among three different side grips. The largest one provides a silicone-coated platform for your pinkie. The other two grip options are smaller but provide different textures.
Lastly, Mad Catz gives you three different wrist rests for the R.A.T. 9. One sits low and is almost flush with the main mouse body. The other two sit higher, but again offer different textures. In addition, you can slide the wrist piece along a well-designed track to make the mouse longer, giving those with larger hands a better grip.
The R.A.T. 9 has a few other features we haven't seen before. Lateral scroll wheels aren't new, but the one on this mouse sits in the lower left corner. We'd rather have it there than not, but we prefer Logitech's design, which integrates lateral scrolling with the primary scroll wheel via a tilting mechanism. The secondary scroll wheel placement is really the only hardware misstep for the R.A.T. 9. Otherwise, we found its performance and overall feel in keeping with a $150 mouse, and its customizability sets a new standard for its market segment.