The surface of the mouse mat is made of fiber polished aluminum, which is made of a special blend according to SteelSeries. Whether this is true I cannot comment on, however, the build quality and finish of the surface is immaculate. I spent almost 30 minutes examining the surface for visible faults, but there were none. The finish is in other words perfect, no bumps, no dents, nothing besides a perfectly flat polished surface.
The base of the pad is made of rubber glued to the aluminum top. The base rubber does a really good job at keeping the mat from moving and in conjunction with the relatively heavy piece of aluminum on top this pad isn't going to move unless you pick it off the table.
Besides being made of rubber, the base also has a thread. It grips the desk like a tire grips a road. As you can on the image above the natural rubber base stops approximately 0.5 cm from the edge of the aluminum surface. This means that you can see the rubber element when the mat is laying on a table even if you look at it from an angle.
The surface can only be described as incredibly smooth. The grain is very fine and has a special feel to it. Even though it's extremely fine grained it maintains a lot of good qualities that normally only a medium rough surface can achieve, i.e. low friction, and control while moving the mouse. Overall the surface seems to be the perfect blend between being fine grained while still maintaining decent gliding performance. As you can see on the pictures above the surface's grain really fine. Besides that the finish of the mat is perfect, even the aluminum edges are filed down so that you don't risk injuring your arm like you could when using the old SteelPads.
The only real problem with the surface is that it feels cold. This is because it's made of aluminum and since it dissipates heat it feels cold to the touch because it "distributes" the heat from your hand. This can causes some condensation water on the surface especially if your room is cold like maybe 19°C. Besides that the surface is so fine that even fingerprints are visible on it.
Above you can see how easy it is to spot thumbs prints on this mat. Of course it isn't something that hurts performance it just doesn't look that pretty.
The SteelSeries SX is a bit thicker than its sibling the S&S. I don't feel that the height of the pad is annoying not even while playing for a really long time. Even though it's a bit thicker it doesn't feel like you have to bend your wrist in an awkward way in order to use the pad.
PerformanceThis is quite hard to judge objectively because almost every gamer has his own opinion on how a mat should be and how it should function. One of the things I look for in a mousepad is that the surface is consistent and performs well with most modern mice. My favorite mouse at the moment is the Razer DeathAdder which uses a 1800 DPI infrared sensor. Besides testing the SteelSeries SX with the DeathAdder I have also used the following mice on it with good results: Microsoft IntelliMouse Explorer 3.0 (Standard optical mouse), OCZ Equalizer (Lazer engine).
Besides having a good surface a mouse mat should also stay in place even while gaming. It's impossible to get a good gaming experience if your mat moves across your desk. These are my main focus areas while testing a mat.
To start up with I launched CounterStrike:Source which is a good fast paced first person shooter game. During the hole test tracking was perfect, I used a fairly low sensitivity in my first run and there was absolutely no stalling or stuttering. I actually enjoyed playing with low sensitivity on this mat because the friction between mat and mouse feet was at a minimum, but still high enough so that you get a sense of control and a bit of feedback from the surface as you maneuver your mouse across it.
After my initial testing in CounterStrike:Source I upped the sensitivity just to test if this particular mat gives enough control of your movement. It was my impression that the balance between glide and control was very good. If you are a high sensitivity gamer you might consider another mat because the SteelPad SX could be considered too slippery for you, but again this all depends on your style of play.
I also tested this mat in Battlefield 2 just to see whether its particular surface fits my playing style. And to my surprise it actually worked quite well. In BF2 you do a lot of quick 180 degree turns and stuff like that and then reset your mouse to the middle of the mat.
This is why I am not so fond of playing with hard mats in BF2 because it takes a split second before the mouse settles in on a hard mat. However I feel that this little problem is balanced out by the superior control gained while using a mat such as the SteelSeries SX. Just like with the SteelSeries S&S the SX has the perfect size for me and the little arch at the bottom makes it quite comfortable to game on.