Notre Dame Basilica is really special and very dear to Montrealers.
In fact many people confuse it with the cathedral Marie-Reine-Des-Coeurs. But they are totally different entities and, in my opinion, cannot compare at all.
In fact there is a kind of historical fight between the two churches. In Montreal the Sulpician priests were not only the priest responsible for the local church but also the local "seigneurs" (landowners of the whole island). The bishop was in Quebec city and the Sulpicians did everything that they could do to prevent the settling of new churches (money talks, in all ways). So the original church (built in early XVIII century, in the middle of Notre Dame Street) was becoming much too small when the population boom happened in the early XIX century.
But the Sulpicians were against construction of of any other church within the city walls, except for the religious order chapels (the only one still standing is Notre-Dame-Du-Bonsecours belonging to the Grey Nuns). As for the rest of the island they wanted only to have "dessertes" (secondary churches) and not real parishes.
So they decided to built this huge church that could contain almost all of the catholic population at once. Gothic revival was becoming popular so they tried to find an architect that was used to the style. They found a protestant architect in New-York James O'Donnell that had a bare knowledge ofthe the new styl: for example he knew what ogival windows were but barely (if you compare to the later Saint-Patrick in New-York, you will see on how much faithful to the originals this cathedral is).
He settled in Montreal and, with the new iron technology + the use of huge wood truncks, he was able to make a very boxy church but with some of the gothic style. He died before the completion of the church (after converting to catholicism and being buried in the church) and was replaced by a another English (and much better architect) John Ostell that did many buildings oll over the island. He was able to complete the plans (particularly the towers) but if O'Donnell had some very clever ideas (like having the floor follow the natural slope of the hill) it was not always the case: for example a rosace window was aligned straight to the east so everybody had to shade their eyes during the morning masses.
The story doesn't end there because the Sulpician asked Ostell to do an expansion of their mission at the "Sault-Au-Recollet", the most beautiful XVII century church on the island and he respected the original decoration which is also magnificient but totally different from Notre Dame. The influx of Irish immigrants also caused a problems and the Sulpicians paid for the Irish Church, Saint Patrick, which is almost as magnificient as Notre-Dame.
But they finally lost their fight with the papacy and the bishop and by the mid 1800's a very strong willed man was named bishop of Montreal and started to built church all over the island, including the cathedral which is a kind of mini ugly Saint-Peter's
But the Notre-Dame history doesn't end there because the Sulpicians decided to have the whole inside decoration redone in the 1880's by a Canadian architect, that was almost self taught but learned to use all the new technological improvements (like the three rosaces in the ceiling made from iron, that brought some light inside) and also use some artificial lighting to highlight the main altar. At the same time an even more over the top chapel was built at the back but, alas it was victim of an arson in the 1980's and the mix of modern and ancient of the reconstruction is not the best.
By the way, this chapel was mainly built because the church was too large for weddings, etc. In fact the last wedding that took place in the main church was that of Celine Dion and even with close to two thousands guests the church looked half empty!.
Finally a word about te ongoing art addition and restauration inside. Even nowadays the church continue to order work of art for its decoration (alas the magnificient art nouveau baptistery by Ozias Leduc is unaccessible). Beside the ordinary tour, it would be a good idea to have a more thouroughly (if more expensive one) to do all the sights (including, close up of the incredible organ). The church still belongs to the Sulpicians and this explains the charge for the visit (but you can request to enter free to pray). This is not a government owned building (like most churches in Europe that were seized by governments in the XIX century). The Sulpicians might have loads of money when they were the "seigneurs" of Montreal but times are ''a changing!
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.