Speaking from the perspective of a part-time Model M user, a wrist rest, at least for me, does not work well at all, especially if used by itself without good forearm/elbow support. A wrist rest, either built in, provided by the manufacturer, or after market, can also lead to bad habits and increased risk of RSI. Ideally the top surface of your hands should be perfectly in line with the top surface of your forearms, i.e. little to no wrist bend vertically, especially tilting the hand up at the wrist. I personally use two Ergo-Rest forearm supports, one mounted on either side of the desk with the keyboard mounted in them middle. When taking the periodic breaks from typing that occur during any sort of work, I either gently rest my hands on the keyboard with some of the weight supported by my hands with the majority supported by the Ergo-Rests. Longer periods of not typing and I will pull my arms away from the desk (Ergo-Rest follows the arms) and let my hands hang loose and free with all the weight supported by the Ergo-Rest. Alternatively, the keyboard can be placed on your lap, and assuming your chair is set up correctly, no wrist support should be required as the chair arms should be at the correct height to support the forearms comparable to the Ergo-Rests for the desktop with rest periods handled more or less the same.
In the image above, the Ergo-Rests are the black curved pads. They are mounted with very smooth pivot points directly under the pads, at the midpoint of the height-adjustable arms, and at the mounting bracket connected to the desk. This allows them to follow and support your arms in a single plane through pretty much the entire range needed for keyboard/trackball/mouse work. They are adjusted so my wrists are as flat as possible when the hands are placed in correct position for typing. I do not use them when typing on the pictured DataHand as the chair arms perform the same function as the Ergo-Rest as is the case when I put a more traditional keyboard such as the Model M on my lap.
