Honestly, I think it really depends on your child. If your child is totally into dress-up and/or princesses, its probably 'worth it.' We've only done the BBB on cruises - and my daughter LOVES it. Personally, we've never done it at the parks because my daughter doesn't really like wearing dresses in the park.
We did do pirates league at the parks and she did the mermaid package. I would recommend that over the BBB at MK. Its a bit less congested and the outfit is a lot more wearable than the princess dresses. She also LOVED that experience.
My daughter is obsessed with dressing up and all things girly. If your child is not, then I wouldn't do it. Its pretty expensive - the time commitment is minimal IMHO. Maybe 45 minutes (?) They are pretty fast.