I was visiting Frankfurt for the book fair and had terrible trouble finding a place to stay, even though I started looking two months ahead. Pension Wiedemann came to my rescue.
Although it is out of the city centre, the pension is very convenient. Raunheim is just two stops on the S-Bahn from Frankfurt airport. And Pension Wiedemann is directly opposite the station exit. It's then just a 25 minute ride into the city centre, with trains every 15 minutes.
The pension is immaculate and cosy. I was given a lovely room complete with sofa and a very comfy bed. There is wi-fi included which functioned perfectly. Breakfast is a buffet with decent choice. The proprietors could not have been more helpful and pleasant.
It's also terrific value for money.