I have them in black for my bodies. Initially they took some getting used to, but now I prefer having them. The early model for the D90 isn't too great, but they've modified how they fit the newer bodies.
There is a piece that slides into the accessory shoe. The silicone piece that fits there tends to come out easily, but I've gotten used to the fact that it happens. The surface of the cover gives me something to grip when I hold the body. Putting on a quick release plate can be problematic. The silicone wraps around the bottom of the body a little although it easily slides out of the way when using the quick release plate. However, if you use a grip, it isn't going to work.
The top and rear of the camera are still exposed to the elements so the cover really only offers rain protection over the black part of the body (which doesn't particularly need it).