Well worth a weekend
I've been going to Frankfurt for years and actually lived there for 6 months. Now I go back a few times a year on quick visits. It's not an obvious tourist city but that has it's advantages. It's a very compact city but feels big and bustling because of the Sky Scrapers giving it the nickname Mainhattan. In terms of sightseeing you could do it all in 2 days but it's a nice city to just to explore and chill in. On the weekend the nightlife is very good and varied.
Insider Tip:The best viewing of the skyline is on the top of Maintower but it costs money and there's often a line. However there a few other free spots to enjoy the skyline: In the city center at Hauptwache you can go up to top of the Zeilgallerie mall and there's a free viewing platform up there. If you want to stick around follow the walkway across the roof to the top of the Kaufhof department store. They have a rooftop cafetiria and balcony you can get a good value meal there or just some drinks and hang out.