The 747 bus is the bus that goes from downtown Montreal (last stop stop Lionel-Groulx metro station) before heading directly to Montreal's Trudeau Airport. I've taken it a few times when I either needed to go to the airport to get some things done as well as taken it when I could not get a lift from a friend or family member either when I came back to Montreal from a flight or when I was going there to catch a flight.
Arriving from the airport and boarding the 747, there isn't much issues since the buses come and stop regularly so the likelihood of getting a seat is very good (if not, you would just wait for the next bus and board that bus). I have a monthly bus/metro pass so my monthly pass allows me to ride the 747 without any additional charges. If you don't have a pass, you could purchase a ticket at the airport which is valid for 24 hours and allows you to take the 747 as well as any bus/metro for 24 hours from the first time you enter the bus/metro. When arriving from the airport, I will generally get off at the first stop which is Lionel-Groulx as this is the closest metro to where I would need to go. I then need to take another metro & bus to get back home. Because of the multiple transfers that I need to get home, generally I will not take this bus if I am coming back from a trip since carrying large luggage is problematic on regular buses depending on how full the bus is. If I didn't require taking another bus after taking the 747 bus to get home, I would probably take the 747 bus more often.
Now, going to the airport, I will generally board at the last stop (Lionel-Groulx) before the bus heads directly to the airport. It can get quite crowded at this stop which is one of the reasons why I won't take it when I'm heading to the airport to travel (but will take it when I'm going to the airport to meet someone or take care of business). Another reason why I won't take this bus to get me to the airport for travel is because Lionel-Groulx is further away from the airport than where I live so it doesn't really make sense to save ~$20 on a cab to go further away from the airport (carrying luggage) to take a direct bus to the airport which will take longer. If I lived along the downtown route of the 747 or I didn't require a bus to get to the metro (in order to get to Lionel-Groulx), I would be more likely to take this bus.
The bus has free WiFi which is great. The ride is fairly quick from Lionel-Groulx metro (maybe 15 minutes). It also has a few downtown stops which is great for travelers staying downtown.
I think that the 747 is a great bus/service for travelers heading to the airport from downtown or going downtown from the airport.