is judo an effective form of combat?
yes , no and maybe.
what it does do is teach you to be able to think/react when things get up close and personal.
it teaches you how to fall.(a very good thing)
what almost any martial art does, is make you realize you can do lots more than you thought you could in a bad situation.
if the art(and compatible teacher) availible is judo, by all means take it.
it is one of the few full speed full contact martial arts.