I got bot the 11mm and 16mm tubes. There was a bit of a deal on them at Amazon for a short time, and I grabbed them both.
The build quality is outstanding. When they are attached, they look and feel like part of the lens.
I have played with them a bot around the house today, but that's it.
They do state that a "malfunction is possible" if they are stacked, but I did try them together with the 55-200, and they seemed to work for that short while.
The 16 is quite happy on my 35. As expected, AF works. It does tend to hunt a bit if you are far from its current focal point depth wise. Once you are in range, it snaps to quite nicely.
It will be interesting to see how the various lenses react IQ wise to them.