The museum is absolutely breathtaking. It sites atop a large staircase, much like the Metropolitan in New York City, but is even more imposing.
We went to see the Dali exhibit, and it was worth the trip. It was a bit crowded, so if you can, go during the day when others are at work. We went early Saturday evening. The exhibit is quite extensive, covering his early and later works, the most popular being, of course, his surrealist period. I would warn that Dali is a bit graphic at times for children, and perhaps for some adults.
The rest of the museum is fabulous, and it would take at least 2 days to do justice to the collection.
One not-to-be-missed area is Duchamp's works, including The Bride Stripped Bare by Her Bachelors and the work to the right of that, behind the wooden door in the corner. Do not miss that one! The museum boasts one of the best Duchamp collections in the US, so don't miss out! (Marcel Duchamp is considered one of the most important high-modern artists. He created seminal works in the surrealist, dada, and cubist styles.)