Hostels really don't get much more conveniently located than Stay and Learn: it's across the road from the train station! It seems that the owners of this hostel are prepared to admit the truth: Frankfurt's a big boring city, and most people only come here in order to make some kind of transport connection (generally a flight). I guess that, due to its name, many people also stay in Frankfurt in order to learn some German — because there's nothing else to do here :P. Considering that it's in big commercial Frankfurt, I found Stay and Learn to be surprisingly fun and colourful, for the one night that I was here. The common room upstairs is very friendly, and the free all-you-can-eat dinner (on some nights) is not to be missed.
Rooms: usual German high standard — quiet, clean and comfortable.
People: friendly and international bunch of backpackers can be found here.
Staff: also very friendly — and available 24/7.
Location: across the road from the train station. The area around the hostel is a red-light district: as well as sex shops, they also have handy junk shops everywhere, and plenty of cheap food.
Food: free dinner on some nights of the week; the Saturday night pasta is excellent. They also have free breakfast, but sadly I had to leave too early to try it.
Internet: numerous PCs available for €0.90/hr; this is the first hostel I've stayed at where the hostel Internet is actually cheaper than the cafe Internet down the road!
Hot water: didn't have time to try it — but the ensuite shower and bathroom in my dorm looked very nice.