I was there back in August 2012 for about 10 days I think. I had booked well in advanced and for…read morethe number of days I stayed it was not expensive.
The room I had was spacious. Double bed, more than enough space to store my luggage and clothes. TV (if you care), free internet, though there was some occasional connection dropout. Desk and armchair. I had the air conditioner too.
The flat is well located, not far from Bockenheimer station (3 minutes' walk), which is great when you have to drag your bag. You can reach the city centre quickly. From the location I could visit Frankfurt and around (Heidelberg, Sanktgoarhausen, Wiesbaden).
The neighbourhood is quiet, at least at night, since I wasn't in during the day. The street lights are no problem. If the curtain is not enough, you can pull down the blinds. It is safe too walking back alone at night (well, I felt so anywhere I went in Germany, as far as I could travel in any case).
The room was always clean (i didn't mess up either, mind you). The receptionists were efficient and welcoming. You do have to pay extra for breakfast. But I always eat outside, even for breakfast, so no big deal. I did try the breakfast once. It was fine and generous enough for me.
So all in all a very pleasant experience.