This place is tres legit. Now, I should warn you that i have a slight fear/fascination of anything having to do with the M. It took us around 2.5 hours to peruse the whole museum. You begin on the top floor, and make your way downstairs to the first floor. The museum is the old courthouse.
Oh! If you have a local with you (with a local ID), they can go for $13.95 (as of 8/8/2017) and you get free parking. Otherwise, I believe parking was $7 or $9. Yikes.
The history is fascinating, and the videos throughout are equally informative. It makes you realize how prevalent it was/is in America's society. I highly recommend this museum; it is different from the M experience, in that you have time to walk through and not be engaged with live characters. The gift shop at the end has fun stuff, too!
I recommend buying your tickets online, so you can save time at the box office.