Allow me to add onto what Martin wrote. The Garden Realm of Dessau-Wörlitz is one of those great sites vitually unknown to foreign tourists. If your route takes you in the vicinity, plan to spend about two hours walking around the grounds. This is not a formal, French style garden, but rather a more naturalistic English style.
Coburg has, in my opinion, Germany's best castle, Veste Coburg, which overlooks this rather attractive small city from a hill.
I will disagree on one point, however. I have found the Autobahn route between Berlin and Munich one of the fastest segments on the whole network. There are, however, several obvious and potentially painful chokepoints, like around Leipzig, Nürnberg and approaching the city of Munich itself. Other than that, however, in several experiences driving A9 and A93 between major cities and interchanges, I could often set the car in cruise control at 130-150 km/hr, and rarely did I need to touch the break. Or perhaps I just got really lucky.
Oh, one last point. By European standards, most German cities are relatively easy to negotiate with a car (even Berlin). Munich, however, is a huge exception. Unless your hotel provides it (and only the most expensive do), you will pay dearly to park, and you may need to park a considerable distance from your hotel. And the city can prove difficult to navigate even with a GPS. If you have nothing planned after Munich besides flying out of Frankfurt, I would suggest ditching the car as soon as you arrive in Munich.
