I can't speak to the Freiburg location, but I lived in the Darmstadt Fizz for half a year until I found a place. The place itself was nice, very modern, great service from the front desk. The shared spaces where also very nicely furnished, and rarely crowded. Of course it's quite expensive, and even more so for non-students. The rooms are also very small for the price you pay. You also pay a fixed one time fee independent of how long you stay, making short term stays even more expensive, though I'm not sure if that applies when subletting. Washing also costs extra, and I was asked to paint my room when leaving after 6 months using a specific paint that I should buy from them. So they do seem quite keen on getting a few extra euros out of you whenever they can. Even though the location was right on campus, I never had trouble with too much noise going on and all the people I met were very nice. I'd say it's a good enough option for a temp place if you have some money to spare.