It may not be Oxford Street or Carnaby Street, but its still someplace that if you like shopping you'll want to check out. For anyone else, it is a great central location where if you can find Rue Sainte-Catherine, you can find your way anyplace else in the city.
That's what I used it for. Whilst I love shopping, these are the same stores that I can find anyplace else and so I used it more as a compass point to navigate the rest of the city. I did like that the portion from Rue Berri to Rue Papineau is shut down to vehicular traffic and so becomes a pedestrian mall with restaurant patios spilling out onto the street and festivals taking over the road.
Make this one of your first stops in the city if you're a tourist and you'll be able to find your way around just fine!