Rue Sainte-Catherine is one of the more popular streets in Montreal. We took a nice long walk along its route. When we reached the sketchy area we turned back to the more touristy area.
Like any other street in a large city, this street has it's good areas and not so good areas. Best to stick to the main part with all the nice shops and restaurants, if you are a tourist.
That's all I've got to say.