Never been ko'd but a good shot to the chin is a few seconds of sharp pain throughout your face leaving a bit of a headache.
I got sparked from behind after a fight a while back. Didn't feel anything just came to a minute later and didn't really know exactly how i had got there. My sense came back to me quickly but i couldn't remember what had happened exactly, though i was hit from behind so i wasn't aware of it at all before it actually occurred. I kept asking my mate what happened on the ride home though, he thought that was pretty funny.
I have never been actually knocked out, but was hurt pretty badly one time while sparring. My opponent hit be high on the head and everything got a little weird, cunfusing, plus it hurt like hell. But a different kind of hurt, a very "deep" hurt that is difficult to explain. Not a superfiscal hurt, say like burning your hand, but a hurt that seems like its a foot deep inside of you. I would imagine getting layed out would be similar, but with a lot more confusion.