While I was still a student I enjoyed this café because you could go out having breakfast without being broke afterwards. And since not everybody in Frankfurt is a stock market broker there are a lot of people who enjoy a good and cheap meal!
If you are really on a tight budget but still need to get your stomach filled, go there. You can eat as much as you want (!) for just a handful of Euros. Of course, the sausage comes from the discounter supermarket and is not bio/organic/gourmet/whatever but still the breakfast tastes good!
They also have a second café opposite of the Frankfurt Zoo! Plus, you can have breakfast all day long until they close the place!