I had been thinking of making my own extension rings after doing some tests with a lens hand-held in front of my camera... Then I saw these tubes... £5.99! It wasn't worth me even trying to make my own!
The tubes are actually made of metal, which came as a bit of a shock. I had expected plastic for this price. No instructions, but it's not exactly rocket science.
Please note, no connections or controls are linked through from the camera. This means you must use the camera on manual, manually focus and manually set the aperture on the lens.
If you have a G lens you are going to be stuck at the smallest aperture and not be able to change it as a G lens has no aperture ring. You could still use it, but you will need a well lit scene to see what you are doing, and a tripod for the longer exposure.
If you have an older lens (I have a Nikon 50mm f/1.8D) the tubes work brilliantly.
I'm a little amazed by those saying that they got the lens stuck etc, there is one "control" on the tubes, the lens release. It's a knurled knob. Honestly, how long does it take to try push, pull, push left, right, up, down? (You push it down towards the camera body). If it's tight, give the lens a little wiggle. I have experienced no problems.
Cons: Not really much not to like. It would be nice of the lens mounting alignment dots were on the side, not on the mounting plate faces, then you would be able to see them as you line thing up, but I think I can live with that... If not, I'm quite capable of putting my own little paint dot on the side!