An extension tube, regardless of brand, does not degrade image quality since it has no lens.
As the name suggests, it is just a tube with no glass in it. (Those with glass are called tele-extenders and have a totally different purpose.)
The difference between brands are connectivity and build quality.
For some third party extension tubes, the lack of the electrical connection means auto focus will not work. Also, since the lens is not connected to the camera, you can't change it's aperture (unless it has an aperture ring).
However even with a high quality extension tube, your camera auto focus will fail 90% of the time anyway, since the DOF is very shallow. So you will almost certainly be manual focusing even with an expensive extension tube.
If your camera body is heavy and your lens is heavy, it only makes sense to use a high quality tube, well-constructed in metal that can take the full weight and ensure the stability and safety of your lens and camera. Some cheap extensive tubes are constructed with thin plastic and may come loose or even snap if you put a heavy load on it. If you are just using a kit lens then it makes little difference. If you are using a heavy and expensive lens then it is worth getting a tube that will not put your expensive gears at risk. After all a lens could cost $800 and a high quality extension tube will only cost $100 or so.
As pointed out by Matt below, it actually does (slightly) affect the image quality. This is due to the small mis-alignment of the lens with the body. It should be rather hard to notice but it is present. Please read his answer for details.