There isn't much left of the palace after arson destroyed it - they started to rebuild it but work was stopped in 2009 after the change in administration due to lacking of funding. As with all national parks/properties in Mada you have to go in with a guide. The guide we had was incredibly knowledgeable and we enjoyed our time with him. We were glad to have gone at the end of our 2 week stay in Mada and the complicated history made more sense than it would have at the beginning of the trip. The guide walks you around the current structure and points out landmarks in the city as the palace is located at the highest point. Our guide also had photos he showed us of what it used to look like in order to serve as a reference as to what we were looking at.
All that being said, be careful with your guide and negotiate the price (additional to the entrance) beforehand. In our time in Mada we always tipped our guides in the parks, the amount determined by us - here the guide essentially insisted on at least a 12-15 euro tip for the visit including the museum, a bit excessive...finally we compromised on the equivalent of 6 euro.
The museum was interesting but there really isn't much there that is original to the palace. Essentially during colonization the French took the expensive stuff back to France and replaced it with things of considerably lower value. So it's well worth it to visit the palace, just be sure to negotiate your guide price in advance.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.