Any free art museum that is remotely decent is 5 stars in my book! Get art to the people! But this museum is way more than remotely decent. It has a pretty large permanent collection for a free art museum. You will see a few famous names, but in general, it will be lesser known artists and work, but just as beautiful. The museum is within several buildings so it can be confusing getting around, and it does take some time to see it.
Great activity on a budget in Montreal, and likely a good activity regardless of your budget.