Absent a train, this is a fairly easy and painless way to get from YUL to downtown. The buses run frequently and there are plenty of ticket machines to buy your pass. The fare is $9 cash or you can buy an all day fare card and then use that on subways and buses the rest of the day.
I like the bus at YYZ a bit better because it's only $3. That takes a star off. The other reason it gets dinged a star is because the bus I rode downtown the other day had no air conditioning! I mean, seriously, where are we? Khartoum? Come on, it's 2013. I expect AC for my $9.
Otherwise, though, a great way to get downtown. I like how they even have space for luggage. Hear that Q33 to LaGuardia?