I stayed at this hostel last year when I ran the Berlin marathon, and at the time I thought this was a fairly decent place, although after staying at the Eastener Hostel on later trips to Berlin, I would not go back and stay at the Hostel Adler again.
On the plus side, the location is perfect and I think it has got to be one of the best placed hostels in Berlin, being only a 10 minute walk or so from Brandenburg Gate and only 5 minutes from the main station of Freidrichstrasse Bahnhof.
On the negative side though this is really all it has going for it. The prices are cheap, but when you see the rooms, you understand why, as the rooms are bare and empty apart from the beds, and could be a lot cleaner. My room still had numerous balloons left from the previous guests.
If you are looking for a hostel with some sort of atmosphere and warmth then please choose somewhere different. This hostel during my stay had absolutely no atmosphere at all, which if travelling alone can be a little depressing.
The hostel also seemed a little on the dirty side, and the bathrooms and toilets could have been much cleaner.
If you are looking somewhere central that's cheap just to sleep then this hostel is okay. If you are looking for anything on top of this then there are plenty of other hostels within the vicinity that offer much more.
For more details check out the following websites:
www.hostel-adler.de,
www.hostel-adler.com