I do like the Canon 12mm tube, the smaller tubes are useful for shorter focal length lenses, and only Canon makes a 12 mm, for more extension, I see no difference between a good set of Kenko Auto extension tubes, and the Canon ones. The Kenko set also allows more combinations of extension for various magnifications.
That's not accurate. Kenko also makes a 12mm, and the automatic extension tube set for Canon (http://www.bhphotovideo.com/c/product/375102-REG/Kenko_AEXTUBEDGC_Auto_Extension_Tube_Set.html) comes with 12mm, 20mm and 36mm tubes that can be used individually or stacked.
Just bought a set of those and they're quite nice. Perhaps not as beefy as the Canon and don't mount quite as smoothly (but fine), but a good cost savings at $180 for up to 68mm in extension.