Stayed a night during a stop over in Frankfurt. Location was great, but finding the hotel took me a little longer as it was (like other mentioned) tucked away in the alley way, where the entrance to the alley way is found between the stores fronts. So it is less than 10 minute walk from the train station.

Likes:
Location from the train station
Affordable
Wifi

Dislikes:
A little sketchy in the red light district
Shared washroom for my accommadation, so one toilet for the floor and thus everyone had a key, which can be used to unlock the door while inside doing your business.

Location: Hotel Adler is conveniently located next to Frankfurt Main Station - although it was hard to find because it's tucked away in an alley. (it's actually right across from Levis Hotel)
Restaurants: There aren't too many around the area, probably have to search around the Main Station to find them.
Breakfast: included but not all that tasty
Service: Front desk is helpful and sincere. They let us store our bags in the lobby on the day we checked out. (The lobby is located on second floor, which is quite strange)
Given the price of the hotel, I would recommend it to people who are traveling on a budget and don't mind the small rooms and small beds. It's close to the red light district, but no harms there as long as you don't walk through the area too late at night.

Myself and a friend shared a twin room for 2 nights. As two men on a city break with wives at home we only wanted something simple and the Hotel Adler met our requirements. It is immaculately clean and the staff, like all the Frankfurters we met were very helpful and friendly. Check-in was efficient but relaxed yet professional. The room itself consisted of two single rooms and an ensuite bathroom and television. Whilst not big it was adequate for our needs. A complimentary bottle of water each day was also refreshing. Tea/Coffee making facilities are downstairs as is breakfast and free internet access. Again straight forward continental breakfast consisting of bread, cereal, cheese and fruit again in a relaxed and pleasant environment. Location wise, nothing particular to say other than it is about a comfortable ten minute walk from the Opera house and twenty minutes walk from the livelier Sachensheusen area. Very close to the train station which was convenient. Others have mentioned proximity to the red light district. This area could be completely avoided en route to the hotel and i felt perfectly safe at all times in Frankfurt and at this hotel. Absolutely no hesitation in recommending this hotel as a great value little hotel with no nonsense. Well done Hotel Adler.

This hotel is ideally located close to the railway station and public transport. Rooms and clean and the breakfast is good. Service was also friendly. There is a free internet (free wireless in rooms and PC in the lobby). If you just want to stay a night, this hotel is practical and recommendable.

However, it is not luxury hotel and the hotel is in fact in a normal apartment building, which has been transformed into a hotel. The halls are not nice.
Travelers should also note that the hotel is very close (200 m) from the Red Light District and Brothels. Depending on you interests, this is either a good or a bad thing, but one should be aware of this fact before reserving.