I've seen more airports than I can likely count, so I am picky and am quickly aggitated by airports which don't make sense.
YUL never made me angry. From the easy transition to ground transport upon arrival, clear signage, and availability of staff in case you have questions, this is an easy airport - I think a novice could easily navigate with ease. Plus one point as well for the proximity of Tim Hortons to the baggage reclaim after my red eye flight.
Even easier is departing - departures terminal again, just makes sense, it's easy to check it with your bags. Even with a large security queue I was through and clear in about 15 minutes when I thought it would be at least 45 minutes.
The airport itself is nice, not as nice as some others, but impeccibly clean and an altogether for being an airport I have no real complaints.