While budget hotels tend to provide the key amenities but have no atmosphere, Hotel Achterbahn is all atmosphere and few amenities. I stayed at 6 budget hotels in German cities on this trip and this was the worst for price (at 40 EUR/night) vs. service, amenities, and location. It's clean and well heated, but it's not good for more than a 2-3 night stay because there's hardly any room to put things. There was no chair or writing table (though there were two drawers) and there was no wardrobe or bar to hang things on, just 4 hangers on pegs on the wall. There was no towel rack in the bathroom(!), just a small knob that the towel kept falling off of. NO bath towel is provided, only hand towels. (And an extra hand towel for decoration only, with a sign instructing you not to use it.) They do provide an extra blanket, which was nice in the middle of winter.
I was on the ground floor and the curtains could be seen through from outside. The lighting is very dim, I gave up before I had even read a page. Really thin walls, I could hear everything going on outside and some of the other guests came back one night at 1 AM shouting and banging on the furniture in the hallway. "Pet friendly" means that if you don't like pets, beware -- if there's a dog in the building you'll hear every bark it makes.
On one day of my stay I left the room at 11 AM and the cleaner never made up the room that day. Checkout is at 10 and at 9 they came to nag me to check out! But otherwise, the staff were friendly.
There is NO INTERNET in the rooms or even at the hotel itself. Free internet means you can use it at Cafe Camera next door, so it's limited to their business hours. They told me I might be able to pick up the signal in my room, but that never happened.
The neighborhood (around the Hauptbahnhof) has a lot of casinos, betting places, strip bars, etc. I don't mind these businesses per se, but in this area they attract a lot of men who stand around in groups on the street day and night. As a female solo traveler I felt uncomfortable walking around here, especially since the sidewalks around this block are otherwise deserted at night. On the other hand, it's a decent location for getting around the city. Marienplatz and the Kunstareal district are a 20-25 minute walk, and the university area and Deutsches Museum are also within walking distance if you're in good shape. The Theriesenwiese U-Bahn is just around the corner, closer than it looks on Google Maps. But note that there are MANY hotels in this area, including a lot that are closer to the center, so shop around.