YUL is a pretty big waste of time. I mean, sure, you get to be in a lovely place like Canada, but only after walking across the entire airport (with only one or two moving walkways). And then when you get to Customs, it's like a free-for-all. The lines wrapped around at least 7 or 8 times, to the point where they held us back and wouldn't let us go downstairs because it was so crowded. The machines they have to process your customs forms are also really, really dumb. The instructions are clear, but for some reason, it made me fill out all of the information for when I was leaving while I was arriving. I guess I'm not going to have to declare anything on the way out? Makes no sense.
Other than that, it's actually a pretty nice looking airport, and there's free (but very spotty) Wi-Fi. There's a currency exchange with pretty lame rates (plus a flat exchange fee of $6 or $7 CAD). The taxi line is fine and moves quickly.
The hardest part is definitely leaving the airport.