If you are visiting more than one museum per day, yes, buy a Paris Museum Pass. The best buy is the 4 day pass-50 euros, which averages 12.50 euros a day, or the 6-day pass, 65 euros, which averages 10.83 euros a day. If you are here 4 days or less and are going to only one museum a day, no, don’t buy one. One of the advantages of the pass is that you bypass the long ticket-buyers line at the big museums. However, you can now buy tickets online in advance for the Louvre, Orsay, Versailles, Pompidou, and the Eiffel Tower (which, not being a museum, is not part of the museum pass) at their websites and still bypass the ticket-buyers line. Children under 18 are admitted free in most museums, so no need to buy one for children.
The other disadvantage of the Paris Museum Pass is that if you want it in advance, it has to be ordered at least 10 days before and it costs 13.50 euros ($17.50) for delivery. The museum pass can also be bought in many museums and at the airports.