I wasn't sure whether or not to buy CityPASS, but I'm really glad I did. We definitely saved money using it, even though we didn't go to the Natural History Museum. I was slightly worried that we wouldn't get to Ellis Island on the day/time we wanted, because you can't pre-book your crossing, but there was no line to exchange the pass for tickets, and we walked straight onto the 10am boat. We used the pass to pre-book Top of the Rock, but that was actually quite complicated to exchange (and to find the entrance). We had also pre-booked the 9/11 museum because we wanted a guided tour on top of entry, and that was easy and recommended. We skipped the lines for Empire State Building, too, but the lines are so much better managed than they used to be, so nobody had to wait long. Highly recommended.