I agree with what most of the other reviewers have said. For me, it's 5 stars for the price and 2 to 3 stars for the product, so 4 stars overall given you buy this in the knowledge it's not top-of-the-range. You can't argue with the price, something this cheap is in the "use once" or "try it to see if you like it before buying something more expensive" bracket. About the product itself, I have a little more to say...
As others have reported, the ring on which you secure the lens is very tight. I've certainly not had the problem of getting lenses stuck to it though, simply twist until the lens is tightly fitted and if it doesn't lock in (which my lens doesn't) then it will certainly be secure enough to hold the lens in place and not let any light in the sides. The other end of the tubes, the part that attaches to your camera body, fits perfectly on my EOS 450d.
The tubes offer great flexibility in magnification with all the various combinations you can attach them in. It's probably too much choice actually, having a couple of rings would be ample for something in this price bracket.
Photographic results are great. Extension tubes have no effect on the quality of your camera output since they contain no optics. Hence, starting off with a cheap set is a decent idea. You are, as others have reported, unable to use any of the electronic features of your lens once the tubes are attached. OK, so lens stabilisation would still work but you would be heartily recommended to turn this off in favour of using a tripod and remote shutter release (or timer) for such detailed work. You can't change the aperture of the lens with the tubes attached which is the real drawback of this product, but again you buy them knowing this will be the case. Again, as others have reported, there are (not recommended by Canon) ways of locking your lens aperture before you attach the tubes. The other electronic lens feature you will have to do without is autofocus, but that's no big problem, you'd probably be better off with manual focus at this sort of magnification anyway.
In conclusion then, clearly not a professional level product, but definitely recommended for the enthusiast on a budget who is just seeding their macro talents or someone who only occasionally might want to take more highly magnified macro shots than are available with their standard lens.