Montreal's airport as come down to many renovations, some are still going on. Monday's are really crowded, since all the business people are leaving. I can not judge the staff since each airline has their own personality.
The airport is relatively clean, can not complain much for that. The mens washroom needed to be cleaned more often was unsanitary.
When going by security to pass customs and security clearance make sure you follow the rules and that your carry-on's are free of liquids, gels, etc... to avoid delays and angry people behind you.
Have a safe trip.