This place is great. I began bringing my son to the Children's Museum when he was 2, and he still loves it at 4. My friends have had a bad experience or two with parents who weren't supervising, but I haven't run into any problems at all.
My son's favorite room right now is the fort room on the 2nd floor. They did it up right. Our tip is to get there as soon as the museum opens -- you'll have the fort room to yourself and you won't have to share your creation right away. Also, they have a few really great books about imagination and fort building in that room.
Just like a few of the other reviewers, we're on our 2nd or 3rd yearly membership, and it's well worth the money. We let our zoo membership lapse, but wouldn't dream of doing so with the Children's Museum.