You'd have to be a real cathedral lover to skip Notre Dame and see only St. Eustache. You really should see both of them as this enormous church was originally built a couple hundred years after Notre Dame to give that cathedral a run for its money. Of course, it hasn't enjoyed the same notoriety as its island cousin (it never had a famous hunchback) - it lacks the same conservation care and hordes of tourists, but nevertheless, St. Eustache served the right bankers well as the parish church for the folks then living at the Louvre and Palais Royal. Indeed, Richelieu, Madame Pompadour and Moliere were all baptised here. Colbert is buried here in a black marble tomb. The cathedral's bulky silhouette is easy to spot when exiting the Les Halles metro station, and when you get closer to the building, you'll notice that its exterior architectural style is not only Gothic (flying buttresses and rose windows) but also Renaissance (the columns) and neoclassical (the later-added facade).
Luckily, the cathedral has interior space of barn-like proportions, allowing for great acoustics (which is what this cathedral has always been known for). Liszt premiered his Grand Mass here as did Berlioz with his Te Deum. The French composer Rameau who is buried here probably still enjoys the musical heritage as organ concerts still take place to this very day (phone 40-26-47-99 for tours and free recital info - Sundays at 5:30 pm).
If the cathedral's name sounds familiar, St. Eustache is also the name of a famous coffee bar in Rome that I reviewed (see my Rome journal). A Roman general, St. Eustache was martyred in a particularly grisly fashion - roasted alive along with his family inside an enormous bronze bull.
www.st-eustache.org