The Kenko Extension Tube Set is very popular because it is a good value and has high functionality. The Kenko set is functionally similar to the Canon Extension Tubes. But with three sizes included in the set (though 25mm is not one of them), the Kenko Extension Tube Set offers more flexibility for similar money.
I searched hard to find a physical difference between the Kenko and the Canon 12mm Extension Tube. What I found is that they are far more similar than they are different.
Probably the only significant downside to the Kenko Extension Tube Set is that it (including the "DG" version) is not Canon EF-S Lens compatible. Update: Multiple people are reporting that the current Kenko Extension Tube version is now EF-S lens compatible - complete with the second lens alignment markings.
The Canon 12mm and the Kenko 12mm extension tubes are identical in weight (2.2 oz) - it is unlikely the one is much stronger than the other as they both feature metal mounts. The other Canon advantages include nicer caps, an included lens pouch and a very slightly more attractive textured finish. Of course the Canon tubes have Canon's name on the side. The Canon tubes have a white mark for EF-S lens alignment which is also missing on the non-EF-S compatible Kenko tubes.
Pictured above is the Canon 12mm Extension Tube with the included pouch (left and center) and the Kenko Extension Tube Set.
Can you tell which of the above tubes is Canon and which is Kenko? The brand differences are obvious in the picture below, but their similarity also shows.