Pretty painless, and the free wifi works well!
When visiting a city, I always am pleasantly surprised when I find that their train system connects right to the airport. Unfortunately that's not the case in Montreal, but the 747 bus helps ameliorate things a little bit.
It runs 24 hours, although I had a fairly long wait late at night. C'est la vie. If you're able, make sure to check the schedule so you're not waiting too long.
It can be a little difficult to find a place to get on between the airport and the gare d'autocars. Being as it's an express bus, stops are limited, but they're also not signed in any particular way. Being as it's a service targeted at travelers you'd think that stops would have "747" in big, bold letters or at least that certain stops would have a pictograph of an airplane or something that could be seen from far away to help locate the stop you need. Additionally, stops in one direction are not necessarily located across the street from the stop going the other way, which can make finding your stop more difficult.