This is a must do! We were in Alaska when they were really active, but it was cloudy the entire time we were there. There's nothing like knowing the northern lights are behind the clouds, but you can't see them :-(.
I have seen it..and more than once..but it is forever at the top of my bucket list. Its not even my favorite thing in Paris..But when you go there,first thing to do - go see the tower. Thats how you believe you are in Paris and not just dreaming ;)