This place has the best dumplings I've ever tried. It's truly exceptional - I liked it so much that on my last visit to Montreal I went out of my way to go there twice. I actually made three meals of it by grabbing takeout on the second trip. (They keep pretty well although the steamed ones reheat to original form in a microwave better than fried.)
The only criticism could be that dumplings are the only thing on the menu, but really I see it as more of a strength. They focus on doing one thing exceptionally well. There is enough variety on their list to keep anyone happy.
Mai Xiang Yuan will be a regular stop on my visits to Montreal from now on.