I know you're technically not supposed to use wrist rests, they are considered a "crutch" and really should be typing with your arms off the desk. But like many people here, I work with less than idea desks, keyboards and not a fan of keyboard trays. For many years, I type with my keyboard on the desk rather than on a lower keyboard tray. I just personally like the up front feeling instead of having my keyboard hidden below the desk. In this typing method, I find that there's more angle between my wrists and the keyboard, especially thicker keyboards.
My first experience in a decent keyboard wrist rest was a attached rest in the Dell keyboards, these were the curve style keyboards they packed in a few years ago. The wrist rest was very well built and just slightly raised the base of my hands enough to keep my arms level with the keyboard. Later I bought a Logitech G15 keyboard and again, had a similar wrist rest, keep in mind both of these are made out of hard plastic, but still comfortable.
A few years later, I started to explore mechanical keyboards, buying a Unicomp keyboard that is a remake of the classic IBM Model M. The keyboard was noticeably taller, and could feel more tension in my wrists from typing but switching to and from the G15 made it a lesser issue. Finally after moving away from the Logitech G15 to save desk space, I bought another mechanical keyboard as my main keyboard, I was looking some a nice wrist rest.
Searching around there were some nice examples, found a few leather wrist rests but the prices were over $70. I just happened to search here on Amazon and found that the 3M wrist rest, looked like the perfect size but wasn't sure about the leatherette material. I decided to buy the wrist rest and test this out with my current setup.
From receiving the wrist rest, the first thing I noticed was the weight, it's more than I expected, which is a good thing, keeps it from moving. Another plus is once you remove the protective shipping plastic film on the bottom, the base is very tacky and grips my desk with no movement. In fact, to adjust the wrist rest, I need to physically pick it up, I can't just slide it back and forth.
The material on the top is actually decently nice, now it's not leather, there's no way it would fool someone, but it's comfortable, easy to wipe off. Also the material doesn't show finger prints or grease as bad as some other materials. Considering some of the leather items I have, this is a plus as your oils and sweat would be difficult to clean off a real leather wrist rest.
The wrist rest is decently firm but you can feel the gel inside with a little push, I would say this is similar to arm chair rest. I personally find it's enough to be comfortable but not so soft that you sink into it, or it's constantly moving. If you have ever used those bean filled rests, you may find them too loose, and this might be a better more stable rest.
From the photos above, you can see that with either a full sized keyboard, or a smaller ten keyless (full sized keyboard with out the number pad on the right), fits well. With the narrower ten keyless keyboard, there's not enough room to use the wrist rest on the keyboard and the for the mouse, but if you were using a "laptop" sized keyboard it might work. I could see this especially with a Netbook, or smaller format laptops.
Overall, I think this is one of the great bargains for making your computer an easier experience. From switching to this wrist rest, I find that typing is much easier and just from gaming, I don't have any issues where my wrist is sore from staying in the same position over longer periods.
As a side note, since I was so happy with this wrist rest I bought the longer version, intending to use this as one piece with the mouse. Personally I like this shorter version as it's the same shape across the entire length, and it's nice to have a second mouse wrist rest that you can adjust differently from the keyboard wrist rest.