I just got the complete Kenko set, and my quick test yesterday was a complete bust with the 70-200VR. The blasted combo acted so weird. The zoom acted like a focus control. No matter which tube I used, the 70-200VR would only focus at one focal length, if at all. It totally blew my mind. I borrowed Patrick's Tamron 90mm macro lens today, and the tubes worked extremely well with it, but I wasn't nearly as happy with the them on the VR.
I plan to get a non-zoom macro lens withing the next few weeks, and I think that they will be very useful for extreme close-up shots.
Hope this helps.
simonramsden said:Auto focus is sooooo slow when you add the tubes, I would stick to MF and use the zoom to adjust image size.
I've just picked up a BR-2A reversing ring, once my step down rings arrive I'm going to mix and match tubes with a reversed 70-200 - just for fun.