Paris does of course have many fine churches, but unlike Poland and Malta most of them tend to be dirty, neglected and in bad repair, not to mention wreckovated. They are indeed a visible sign of the state of French Catholicism. However there are some exceptions to the rule. Here is Saint Trinite, where adoration was going on.
I went to mass in Saint Eugene by the conservatoire, a nineteenth century church below where they have high mass in the 1962 missal. It attracts a respectable crowd, but one feels that there could be more. For vespers I went to St. Nicolas du Chardonnets, the SSPX church. It seems to be the only genuinely Catholic church in Paris.
The Cathedral of Notre Dame of course needs no introduction. It is more crowded with tourists rather than worshippers however.