Pros:
This is a really great location. From what we could tell, it is a very safe area. You're about 10-15 minutes walking to the heart of the old city and the attractions, and you are one short block from the park. You have immediate access to the restaurants on Grande Allée, but even better, you're 10 minutes walking from Cartier Ave and Rue Saint-Jean, which I think are much nicer for eating out than Grande Allée or the old city. The unit itself is very nice. The host has invested in excellent showers, comfortable beds, and top of the line Mitsubishi AC split units in both bedrooms. The unit was clean except for an piece of popcorn and a Gatorade bottle that we found under a bed when we looked under it on checkout. Overall, we were very happy with our stay. We did not do any cooking, but it was great to have the tables, microwave, toaster, and dishwasher. (We didn't use the coffee machine.) The host was always super fast with responses via the Airbnb app and wifi worked great.
I didn't understand the layout from the photos, and so I'll explain it here to help future guests. You walk up outside to the second floor, and then inside on the second floor, you have access to a secure door that takes you up to the third floor. No one else has access to the whole third floor. Up on the third floor, there are two bedrooms and a sitting area in the hallway. Access to the terrace is via a locked door that you control, but other people could also access the terrace via the fire escape. The bedrooms themselves do not have locks on their doors, but since the whole unit is secure, they are not needed.
Cons:
As others have mentioned, you are near Grand Allee, and on Thursday, Friday, and Saturday nights the street is closed off to make a pedestrian area (at least in August it is). Unfortunately, this routes a lot of traffic around the unit, and when the weather is nice, there is also a lot of people noise late into the night. The bedroom with the two beds seems to have better window seals and is a lot quieter than the room with a single bed. Definitely check to make sure the windows are closed at the tops, but even so, it doesn't seem like the windows in the room with a single bed close completely and sounds leaks in. We found that running the HVAC fan helped a lot. I also had to use earplugs to get a sound sleep, but I'm used to 100% silence where I live. The other nights, there were no noise issues.
The other con is having to figure out parking if you've come via car. There are a lot of underground garages a short walk from the unit, but we found the outdoor parking at the Plains of Abraham lot (655 Wilfrid-Laurier Avenue) to work best for us. To use it, you can pay for 24 hours and print out a ticket to display in your dash. This lets you come and go, but beware that the attendant warned us that M-F during the day, office workers also want to use the lot and if you leave during the day, you might not find a spot again until evening. We largely walked everywhere, and so we just had to visit the lot and put a fresh ticket out every day.
While the door to the terrace has a deadbolt, the door on the second floor does not, but the door and lock seem very secure and high quality. The lock is not cheap or flimsy from what I could tell, but even so, I'd prefer a deadbolt on all exterior doors. Overall, I felt safe.
The terrace isn't shaded, and we were a bit nervous about the power lines overhead, and didn't really want to sit next to the air conditioners, and so we didn't make use of the terrace. For some people in cooler weather, I'm sure the terrace is a nice feature.
Summary: We highly recommend the host and unit.