It's an airport. . . it's fine. Very clean and compact. Pretty easy to get around and not overrun with people like NYC area airports. Best part is the easy, cheap, and quick access to/from downtown Montreal via the 747 bus line.
At this airport, you do US customs in Montreal before you arrive in the states. There is good and bad to this. Good, is that once you arrive in the US, you don't have to allocate extra time for customs. Bad, is that you don't get a stamp in your passport! Bummer!
Otherwise, this airport gets a solid 4 stars.