For me it’s hit or miss. The Mona Lisa was underwhelming in person, and I saw it on a day when there were few crowds. Guernica, the Sistine Chapel (if you avoid the cattle prods), Garden of Earthly Delights (and plenty of the Goya/Velazquez/El Greco works at the Peadon), the Van Gogh room in Musee d’Orsay —- all worth a trip if you have the means.