Built as a vanity project of a Roman Catholic Monseigneur in the late 1890s, it is a replica of St. Peter's Basilica in Rome (both inside and out). If you've never seen St. Peter's, then it's really impressive to see, but you won't be awed if you've seen the real thing in Rome. This cathedral has been restored in the past and has ongoing restorations, so some of it may be off limits when you go. It's got some grade A religious art and sculpture, so if you are an art lover, this is great place to see some up close. Be sure to pick up a guide/brochure detailing the history, art and architecture on the columns near the back of the nave. (You might have to look for it.) The tour of the church is better with it.
It's located in downtown Montreal and is within walking distance of the Bonaventure Metro stop. You can access the metro through the basement of the Fairmont Queen Elizabeth next door. Like all churches, it's wheelchair accessible. The cathedral is open throughout the day and there is no admission fee, but it's recommended that you give a donation for the general upkeep.
It's worth a visit if you are an art or architecture fan, but you aren't going to regret missing it if you have limited time in Montreal or have already seen St. Peter's in Rome.