I got the Canon versions for the same reason as Neuro, for use with my 600mm, but from what I hear and read, the Kenko will work just as well. Giving the short DOF, I strongly recommend a macro focusing slider and the best I have found is the Really Right Stuff. It is very high quality, very smooth. For really close ups I use the Novoflex bellow. But then your mobility become quite snailish.
Any thoughts on the Novoflex slider?
Novoflex in general delivers very high quality products. I have not used their slider, but I am sure it´s a very good alternative. It also has some expansion options which could be of interest if this is your main area of photography. To me macro is a fun add-on in the absence of birds and wildlife.
I have been very happy with the RRS heads I have for my tripods and monopod and the solid and simplistic design of their slider appealed to me. The micro focus control you get with the lead screw (1.25mm pr. 360 degree turn) works very well. It is also $200 cheaper than the Novoflex.
1DXII, 5DIV, 5DSR, 8-15/4L, 11-24/4L, 16-35/2.8L III, 24-70/28L II, 70-200/2.8L IS II, 100-400/4.5-5.6L IS II, 200-400/4L IS 1.4x, 17/3.5L TS-E, 24/4L TS-E, 35/1.4L II, 100/2.8L IS Macro, 300/2.8L IS II, 600/4L IS II, Zeiss: 15/2.8, 35/1.4, 55/1.4 Otus, 85/1.4 Otus, Profoto B1