First off, I have to admit I'm a little biased when it comes to Frankfurt (and Germany in general). I was born and spent most of my childhood there. Frankfurt on its own is not the prettiest city in Germany, but the village where I grew up is beautiful. I'm not saying its not worth the trip to Frankfurt, it is just very business oriented (the banking capital of Europe) and very grey and full of concrete buildings. As with most european cities, it does have its history and its charm, and a wonderful river that runs through it.
This brings me to the town where I grew up. Imagine the typical German town in your mind, that's pretty much what it looked like. Old castle, cathedral, ancient houses, beautiful gardens.
A bit of the village
This is the castle and church in the background. These days it stands primarily empty and houses museums and some municipal offices.