Ron, thanks for the good summary.
I would say this about even the cheap optical adapters: I've used one for special needs and it worked. Near as I could tell, it had no more effect on the image quality than a teleconverter. It's not something I'd want to use routinely, but if I had a really expensive and specialized FD L-series lens, I'd use it in a heartbeat. I'm thinking of lenses like the old flourite telephotos, such as the 300/2.8 with extender, that would be expensive to replace today. Those lenses utterly pushed the state of the art and were Canon's entry into the sports photo market that they now dominate.
I would not, of course, buy the optical adaptor as a step in collecting old FD lenses. It only has value for those who have the lenses already and want to keep using them for a while.
Rick "who has a range of FD lenses, but no flourite telephoto, heh, heh" Denney