mh well. I guess its a nice city to see, a good place to go out at night, see the skyline, in the summer its nice to sit next to the main or drink some applewine and eat typicall food from the region. In my opinion Berlin, hamburg and especially munich show more attractive places than frankfurt. but as almost every city it has its own "character". So if you only plan to visit one city, you should go and see the region around Munich. Or Berlin.