I'm slowly watching through all the Star Trek series for the first time right now, and I recently watched the first season of DS9. Even just the first season struck me with how different it felt to TNG, but in a good way. I really enjoyed it. It felt similar to Babylon 5, with the limited environment and the tackling of political issues. They had some good treatments of various ideas as well, such as the war criminal episode. I had always had the impression, which you mentioned, that DS9 was the ugly cousin of the Star Trek family, but I enjoyed the first season quite a bit and am looking forward to watching the rest.