As you've likely noticed, canisters aren't that popular around here, although I know some people have used them. I think they're mentioned in one of the octopus care articles, and I seem to remember that Roy Caldwell (neognodactylus) has mentioned he uses them in his lab setup. The main thing is that manufacturer ratings don't take into account the huge amount of waste produced by cephs, so you'll want to get one rated much larger than the actual size of the tank, and you may have to change the filter media more often.
I really only know this from trying to read a lot here, though, so hopefully some people with experience will chime in shortly...