Paris is beautiful. Any weather, any season. In May when everything blossoms particularly (that's when I recommend you to go: it's nice and warm but there is freshness in the air.
I always thought it's been a bit overrated as the city until I visited it myself. It has it all ready and waiting for you if you know how to enjoy the life - best cuisine, fine architecture, famous fashion, great weather and european chic. One of the best wines I drunk in my life are from Paris too :) as well as tastiest gourmet food, be it a small cafe at the corner or posh the avenue des Champs-Elysées restaurant.
Paris is charming but you don't find here London's efficiency, I am afraid - the tube map is quite tricky to read and if you plan buy out the actual Louvre tickets you paid for online, you might find the place could be closed or the owner went to have a cup of coffee in sunny haze somewhere and unlikely comes back soon. You just got to enjoy it the way it is. This is the thing about Paris, it's not formal a tiny bit - it's all about going with the flow and enjoy every moment.