Summary: I wanted to use it with my Canon 2.8/70-200 IS lens for working distances up to about 5 feet. It is somewhat disappointing to find out that autofocusing is virtually impossible, although an other reviewer claimed to use it successfully with this lens. Later I found in the instructions to set the lens to manual focus. If I would have known this before, I would not have bought it.
Pros: Durable, Easily Interchangeable, Rugged, Strong Construction
Summary: This is a quick and simple way to use any lens for psuedo macro work and it's a lot lighter to carry around then another lens. I find this extension tube length reduces the focus distance enough that an object that normally fills roughly 1/6th of my 1D MkIII viewfinder will take up about 3/4 of the viewfinder. The build quality is excellent as should be expected at this price.
Pros: Excellent Build Quality, Fits perfectly, Light and small to carry
Summary: I bought this to use with the lens I have rather than buying a macro lens at this time. I have used it mostly with my 300 lens on flowers and birds. I have been able to expand my ability to take close up shots that I otherwise would not have been able to get. It is very easy to use, it only takes a second to put it on or take it off the lens that I'm using. I have found it is handy to have in my bag, you never know when you are going to need it.
Summary: I'm using this extension tube primarily with my Canon EF 70-200 f4L IS lens. The images are stunning and the extension tube/lens combination is easy to use. If you've already got some excellent lenses and don't need magnification beyond 1-to-1, then this is an inexpensive solution to seriously consider, particularly as compared to buying a dedicated macro lens.
Summary: I use this primarily on a 300 f/4. The minimum focus distance is reduced by about 20%. Also provides a slight increase in magnification. The maximum focus distance is reduced to about 12 yards.