I bought one of these and it killed my Canon EOS 20D!

If you are thinking about getting a AF confirming adapter, please read this before you decide one way or the other.

The idea of a AF confirmation from the finder in the APS sized sensor Canon cameras is very tempting since the focus screen is not very large or bright, and when using Nikkor or other manual focus lenses, it may be difficult to be sure of really sharp focus. However, when I attached the device to my Nikkor lens and put it on the Canon camera, the battery showed as low power, then Error 99. This was followed by shorting out the circuits so the camera would not turn on, period.
The Canon approved repair facility confirmed that the electronics in the lens mount and the main board of the 20D were damaged and needed to be replaced (ca $350). It is clear that the adapter did this. I had, before testing the Nikon to EOS AF mount bought a similar M42>EOS AF confirm mount. With some trepidation, I tried it on my old Canon EOS 650 film camera ($30), and while it didn't work on that body at least, it also didn't short anything out.

 Addendum 1 (3 February 2007)
I have posted notices on this matter on American and English forums, and so far I seem to be one of the few persons who has had such an experience. Most of the others that had problems did not have as complete a failure as I did. This is good news for everyone, I guess, though I'd still recommend caution. At the time of this addendum, I still didn't have my camera back from repair, but see below.
Addendum 2 (14 February 2007)
The camera came back from the Canon repair facility with new lens mount and circuits, worked for one picture in store. When I got it home and tried to take a picture, with the same lens, flash card, etc. as in the store--the camera gave an "err 99" message and died again. Took it back to store, with another lens on it and a different card, said, no CF -- Went back to the repair people again.

Addendum (23 April 2007)
The Canon authorized repair people did yeoman service (after not quite getting it the first time round) and repaired the camera and got it back to me in time for a very important trip. They did confirm that the adapter, which I had sent along, was the cause of the failure. The repaired camera worked fine, though I had bought an XTi as a backup, just in case. I will not be mounting the Nikon-EOS adapter for a second try however. Most people using these have been much luckier, but I have decided to let my tired old eyes do the focus confirmation from now on. Think about what these are. The makers have pirated a little bit of code/circuitry from a Canon EF lens and pasted this bit of circuitry on a normal X to EOS adapter. If they make a crummy circuit, paste it on in the wrong place, whatever-- I can testify that it will potentially do great harm.

My recommendation-- I personally will not use one of these confirmation adapters again. It fried pretty much every circuit in my 20D. The slight advantage of getting focus confirmation is for me at least offset by the damage that at least one user (ME) has incurred. I would, however, appreciate any feedback from any of you on your experiences with these, good or bad and I'll pass it on.

I will note in this revision (2007-9-2) that the new Canon 40D has a larger and brighter viewfinder together with officially interchangeable screens for better focusing, etc.