with the type of camera you want it for.
Here is a little review of these tubes: Though I haven't had a chance to use them extensively, they seem like they do exactly what they are meant to do. That is, they move the actual lens further away from the sensor (you will need a lens in order to use these tubes) and thus create a larger representation of your subject on the sensor.
The extension tubes fit nicely onto the camera body-turn until it clicks- (or at least nicely onto the Canon camera body) and then the actual lens turns and clicks right onto the end of the tubes. The tubes come in a set of three, all of which can be stacked upon themselves, each additional tube moves the lens further from the sensor. Each individual tube in my set has a number on it, 1, 2 or 3 (besides that there is no writing, branding or instructions)...and when all are stacked upon themselves the tube becomes approximately three inches long (the width is the same as the width of the connecting ring on the camera body).
When connected, shooting manually makes the most sense. You lose control over your aperture due to the fact that no electrical signals are transferred through the mostly metal tubing. As limiting as this is, macro photography seems to be a style of photography that almost demands the use of a tripod. This allows you to have more freedom with your shutter speed, and makes it and your ISO your main methods of controlling lighting. Flash can also help, though the built in on-camera flash does not seem to work for what I want...I would definitely recommend off-camera flash when possible. Macro ring flashes also seem to work wonders. I do not (yet) have access to one, so I can not say for sure how they would work with these extension tubes, though I really can't see what problems they would pose.
Image quality seems rather good. Especially for the fact that the set cost me less than $7. I don't know if having your lens further away from the sensor would affect the image (it definitely does open your camera body to potential dust (try to brush out the tubes before connecting them...anything in them could make it's way into the camera)) It doesn't seem like the image was affected...My results were exactly what I would expect from Canon's "Nifty Fifty," just with a much closer image than I could normally get with the 50mm.
All in all I really think this is worth the $7. Definitely. It allows you to change the distance between your sensor and lens (three different possible distances) giving you the "macro" look without having to dish out big bucks for a lens specific for this style of photography. Not dishing out said big bucks means you don't get control over your aperture, but for me, that is a small price to pay. It is likely that if something like these extension tubes didn't exist, I would never get to try macro photography. This allows me to test the waters, see just how much I like it, and hey, if I hate it I can throw them away without feeling like I just lost any money, let alone significant money. Give them a go...what could you possibly lose?
Like I said, the subject wasn't nearly as important as the proximity to it. I grabbed the first thing that I thought might look interesting macro-ized. All of these pictures were shot using a 50mm lens (on my T1i, so figure in the 1.6 crop factor) and I had all three extension tubes between my lens and camera. The final images were all cropped to some extent.