Pros: The extensions are modular, with three pieces purely for extension and two adaptor pieces, one for the camera and the other for the objective. As other reviewers noted, the tubes are metallic, with a good mount (I didn't have metal sparks in mine) and a switch for removing the objective when you're done with it. I didn't have removal problems, residues nor did I notice any defects to the camera.

Cons: Not really, but if I do have to find any, I would say that due to the lack of electronic interface, you have to preset the aperture prior to mounting the extensions. This is something everyone with this equipment has already noted, and is a minor consequence (I think) of the ultra-reduced price.

Other Thoughts:

Bottomline: This is a product I heavily recommend, as opposed to any dioptric lenses, which have poor optical quality. This is just a trick to bring the focus point from very close objects right into the sensor. Does not have any "light-processing" parts.

Pros: Cheap
Metal, not plastic

Cons: Fit to camera is either too loose or too tight (see below)
May ruin your camera (see below)
Poor machining, rings fit each other, but it sounds and feels like there's sand everywhere
Gives a very low span of focus, in the case of my 50mm lens, what used to be a span of focus length from about 50 cm to infinity was now snapped only a few centimeters from max to min focus length. I don't have experience with real macro lenses, but I assume they don't have this problem.

Other Thoughts: When fitting the inner ring to my digital EOS camera, the fit was very loose, and I had to be very careful when moving the camera around so that the lens+rings didn't come off and fall to the ground. Tired of this I decided to try to fit it tightly. It snapped on properly, but this time the fit was so tight I could not get the inner ring off my camera. Believe me I've tried. So now I have a $600 camera with a fairly useless macro ring stuck to it...

Bottomline: Avoid unless you want to ruin your camera!

Pros: - The number of rings. Unlike a previous reviewer, I love the flexibility afforded by them. You can use them on their own, or in combination. Very handy if you're using a prime lens.
- The build quality is quite a bit better than comparably priced eBay stuff that I've seen.

Cons: - The rings weren't finished properly. Mine came with metal burrs that weren't polished off. If you're using the cheaper plastic lenses, it may well have scratched it up.
- No instructions on how to attach the lens, but it shouldn't be too difficult to figure out.

Other Thoughts:

Bottomline: You need to set the f-stop before mounting the lens on the tube, and you'll need to focus manually. But still, it's a LOT cheaper than buying a macro lens.
Overall, a great inexpensive way to get into macro photography.

Pros: I didn't know the first thing about macro photography, which is what made this attractive.
It's solid metal, and the rings assemble/disassemble easy enough to change the length of the tube without a problem.
I played with this using the kit 18-55mm lens that came with my camera, and was impressed at how close I could now bring the camera to the subject being shot.

Cons: - no electrical connection, so you're on your own for focus etc - but I really didn't find it to be much of an issue

Other Thoughts: I've seen really bad movies for more than this cost. A fun thing to try, and will probably even be useful at some point.

Bottomline: cheap thrills with a tube