I have had 2 bad ones: One from snowboarding (main reason why I don't want to try to snowboard again) and the other from dirtbike racing. One mild one from skiing when I why overshot a jump. With the bad ones I blacked out for about 15 mins and with all of them I would have a rather bad headache for up to a week.
Concussions are serious business though. I watched a special about former athletes such as boxers and football players would experienced way to many concussions in their career and as they got older many of them could no longer function on thier own or became braindead.