There are plenty of reports, just on FlyerTalk, of people being turned away at the border or sent straight back on the next available flight for having DUI/DWI convictions. It's not a theoretical "maybe", like a lot of discussions are on here. It definitely happens, and seems to happen fairly regularly.

What I can personally confirm, having been sent to Immigration secondary twice myself and traveling with three other guys who got an Immigration secondary, is that the Canadians are very, very touchy. These were Immigration secondaries, not Customs, which I've never had to visit and personally know of no one else who has.

It can be a pain to enter Canada on business. Oh, and shipping stuff in? I've had more shipping issues with stuff going to Canada than I have with any other country besides Brasil and Russia.