This is more of an equipment recommendation question than a technical one. I am concerned that the extension may degrade the image quality. I guess my question is two fold.
How do extension tubes work?
Can a good quality extension tube degrade the image quality?
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.
Extension tubes work by moving a lens further from the film or sensor.
The closer an object is to the lens, the further back the focussed image will appear, hence extension tubes allow you to focus on objects closer to the lens than would be possible otherwise. They are commonly used a cheap way to improve the macro abilities of a lens.
There are a few ways image quality could be compromised
Floating elements. Some lenses include a "floating" element (just a lens that moves independently of the focussing group) in order to reduce aberrations when focussing close up. If you set the lens focus to a distant setting and then use extension tubes to obtain close focus then you wont be getting the benefit of the floating element and image quality can suffer.
Misalignment of the lens. A certain amount of misalignment is inevitable though most cases it will not have a noticeable impact on image quality. In extreme cases this could manifest itself in a tilt in the plane of focus, e.g. the right side of the image might be in focus but the left side out of focus.
Light leakage or flare (unlikely unless the tube is very badly made, though possible for a DIY effort).
Glare can be a real problem even if the inner surface is black, but not matt. For really bad tubes like those branded COMIX, some glare is present almost always. If a strong light source is outside the image frame, but near its edge, image quality can suffer dramatically, and automatic exposure and white balance sensors also be "confused".
Air is air, but by changing the sensor/film to lens distance a lot of light will bounce from the inner walls unless they are ribbed and with a completely flat black finish. Take a look at the photographs in my review comparing three "equivalent" sets of automatic extension tubes for MFT cameras. The review is in three instalments: part 1, part 2 and part 3. I bet you will never ever again think that lack of "glass" could mean that degradation of image quality will be always similar between different brands and qualities of simple tubes. This of course also applies to lens mount adapters.
In addition to image quality issues related to misalignment or poor manufacturing (light leakage or reflections), there is another potential source of image degradation when using extension tubes: the flaws in the lens attached to the tube will be magnified.
By moving the lens further from the image plane, the tube has the effect of magnifying the center part of the image circle cast by the lens to cover the entire sensor instead of just covering the central portion of the sensor. Any optical defects in the original lens that involve the center of the image circle will be spread over the entire sensor and will be easier to see when the image is viewed.
There is no difference in IQ between different extension tubes directly. There is no glass, the quality loss comes from the change in focusing distance which can lead to an impact on sharpness and focal distance characteristics of the lens.
The only way I can think of that the quality might be impacted is that the camera might no less about what is going on and have a harder time adjusting, so automatic modes might be impacted, but generally manual is suggested for working with extension tubes anyway, in which case the impact would be uniform.
