We came here twice last year in one weekend. We originally came here on Saturday evening for a work party. Let me say that as a party hosting venue, the ROM is spectacular. We had the main room and the entrance/lobby to ourselves for a closed event and it was amazing. How often do you get to hang out and have drinks with a 16th century Buddha and several of your closest dinosaur friends? Almost never.
We came back the following day to tour the museum itself, which was absolutely fascinating. While my wife and I aren't exactly museum people, we still appreciated the history and beauty of the exhibits.
Our entrance to the museum was part of a day pass that included entrance to the aquarium, the CN tower, and some other things. If you have a day to site see, I recommend getting a pass and visiting as many of those locations as you can!