First, the building is stunning.
But there is on the third floor an exhibit dealing generally with the transformation of the German legal system during the Third Reich and particularly with the prosecution of those like Sophie and Hans Scholl in the White Rose resistance movement. It is not big--you can do this in 30 minutes--but it is exceptionally instrructive and moving.
And the Nazi Documentation Museum nearby is absolutely first-rate.
The reason that I am highlighting the courthouse exhibit is that when I went (early), I was the only visitor though I saw a few entering the building as I was leaving.