I stayed in the Gustav Stresemann Institute as part of a conference group. The Institute itself is pretty easy to find, just a few stops from Bonn HBF by metro and a short walk from the station along a residential street.
The good things about GSI were really pretty good. Firstly it had the best wifi internet I've had in a hotel - you can tell it's been set up mostly for business. The institute itself was thoughtfully laid out around a central very pleasant green area which was ideal for informal meetings and for lunch when the weather permitted. The conference space was also really nice, one huge room that subdivided very well into smaller rooms, it was also lovely and bright which really makes a difference in a plenary space. And there was a nice lobby area. I did hear there was also a good bar downstairs although I didn't get to see it. Food was also pretty good albeit I think a bit expensive and there was a great range of foods for breakfast. And the staff were mostly pretty nice as well. I liked the bathroom too, it was more like a basic kind of wetroom and the shower was very good.
On the less good side, there were a few issues. The rooms were tiny, though the communal spaces were so good that this didn't matter so much. The bigger issue was that the walls were paper thin. On the first night someone tried to get something out of their wardrobe in the room next door and I woke thinking someone was kicking the door in. People were mostly very thoughtful about not making noise - the person next door had set their mobile phone alarm on vibrate - but given the lack of soundproofing, it felt like it was my phone. It was very quiet through the night though.
The other minor issue for me was the weird smoking room in the lobby of the institute which was really pretty unpleasant since it seemed to keep letting blasts of cigarette smoke into the lobby.
I think it was a good space for a conference and would recommend it for this purpose but if a good night's sleep is important to you I'd say investigate staying in one of the nearby hotels.