Museums

When my darling brother and I were in Paris last summer, we had a wonderful time running around to museums in the day, and eating expensive and delicious meals by night.  The expensive and delicious (for two teenagers) meals were funded by the fact that we had to pay nothing by day.  When you are under eighteen, the most beautiful art and buildings in the world open up for you for free.  So it didn't matter that we accidentally got into the Conciergerie when we were trying to get to Saint-Chapelle because we weren't the ones paying 14 Euros.  It didn't matter if we got to the Louvre forty minutes before it closed because we weren't paying anything.  We just ran around for free.
Now that I'm eighteen, I've been in the city for something like a little more than two weeks and I haven't been to one museum.  I can make excuses for this - I was signing up for classes, it's cold out, now I have classes (French for three hours every evening...) and lectures and stuff to study for - but a lot of it's because I can't just waltz into the Musée d'Orsay to see the polar bear and the Art Nouveau rooms and run out again.
It turns out however, that you can get these special cards for 15 Euros apiece that will let you have free entrance to the Musée d'Orsay or to the Louvre for a year.  But I still love the Musée Rodin and there are so many museums I still want to see!  The other option for those museums I guess is to say that I'm my brother's twin so that we can go in free.... he looks older than me, anyways.  But that's illegal and wrong...
Have you been to Paris and its museums?  Did they seem expensive to you?