Do lens mount adapters work completely fine?

Question

Obviously besides being able to use any electronic signals to your camera like with canon EOS lenses, is there any downside to using a mount adapter for lenses?

I realize of course I wouldnt be able to use an EF (EOS) mount lens on a differnt type of mount, But i have seen for example Nikon F mount to EF adapter on e-bay and other places. I am just wondering if there is quality loss for some thing like that.

Answer 1

http://www.photoethnography.com/ClassicCameras/Lens-adapters.html

http://www.lesbosher.co.uk/lensadapt.htm
http://photonotes.org/manual-lenses.html
http://photonotes.org/manual-lenses.html
Good luck