For the uninitiated, Wikipedia has this to say about Extension Tubes:
An extension tube is an accessory for cameras with interchangeable lenses, used primarily for macro photography. The tube contains no optical elements; its sole purpose is to move the lens farther from the image plane. The farther away the lens is, the closer the focus, the greater the magnification.....
I don't mind using a cheap extension tube because it's just a hollow tube and has no effect on the quality of the image produced. The real disadvantage of this set and the reason they're at least £100 less expensive than the next set (and many hundreds of pounds cheaper than the Canon equivalent) is there are no electrical contacts, they're simply plain tubes. This means you're not able to control the aperture of the lens and must use manual focus; manual focus being no biggie since that's advisable with such a small depth of field.
Here are my first test images to see how they perform (click to see larger versions over on Flickr):
Both of these images were taken using my kit lens at 55mm and F5.6 hence the depth of field is so shallow on the magnified image with the tubes attached which also had a lot longer exposure in order to expose the image correctly.
For greater depth of field there are ways, not recommended by Canon I should add, you can change and lock the aperture of the lens before you add the extension tubes. This means if you're really desperate there's nothing stopping you getting a much better depth of field with these tubes at the expense of even longer exposure times of course.
For me, I think I shall experiment with these a little more. Should I catch the macro photography bug then I'll have to make a decision about whether to get a better set of extension tubes or even at some point treat myself to a proper macro lens. For now though, these will suffice quite nicely.
EDIT: My Amazon Review has now been published.