I really like Heidelberg and don't understand at all why Rick slams it. The castle is beautiful, the view from there is gorgeous, and the Apotheke Museum that comes with your ticket to the castle and funicular is actually very interesting. Riding the funicular is a fun trip in itself.
The 2 main churches are well worth some time and I find the Jesuit church to be truly elegant, with clean lines and colors. Go in the garden off to the side.
Make sure to visit the Student Prison and the Aula of the university. I went to Heidelberg many times before visiting the Student prison, not knowing what a unique place this is. There are also many nice side streets to explore, with cafes, and even a Kathe Wohlfart store.
Mark Twain knew what he was talking about when he praised Heidelberg. If it was good enough for him, it is good enough for me.
We will discuss Mainz and Wiesbaden some other time, they get the same slamming treatment in the books.