Each time I've gone to Liberty Science Center over the past 10 years (took the kids when they were really young, accompanied a class trip for 3rd graders), I've always come away feeling like: "That's it?" It always seems like there should be more stuff than there is. I've had much better impressions from the Franklin Institute, and also from the Boston Childrens' Museum. Unfortunately, LSC always comes in last when comparing to other equivalent museums... I want to like LSC, but come away feeling like I spent too much on admission, and had to pay for parking too!
However, I agree with the above consensus: under age 10, and you are fine, over 10 and they'll resent you for wasting their time!