It's fairly small and nothing fancy, but if you want to find out about the history of Slovenia, this is a must see. It effectively tells the story of the country from WW I when the Austro-Hungarian Empire broke up to the present day, including all of the important milestones along the way, including life under Tito's communism, and independence. There's a lot to read and a lot to think about. The employee sitting at the entrance to the main second door display was very helpful; I had questions about Slovenia and some of the information in the displays. The employee spoke excellent English and we had a detailed and friendly conversation about several aspects of Slovenia's history.
I was there on a Sunday afternoon, and it was uncrowded. It's also inexpensive; the ticket prices are reasonable, and even the drinks in the vending machine on the first floor were cheap. It's fairly close to the entrance of Tivoli Park.