Star Trek: Deep Space Nine was the last Trek I watched all the way through (I didn't have cable when Voyager and Enterprise came out, and neither could be seen well on my local stations). When I first watched it, back when it started, I felt it was decent, but not up there with Next Generation. Now that I'm rewatching it, however, I feel that it's actually better than Next Gen.
I'm rewatching it from both ends at once (from the beginning with my daughter-- we just started season 2-- and from the end by myself; I've watched most of seasons 6 and 7), so my perceptions may be a bit odd. But what I like about it is that it isn't a copy of the original Trek. It's the first Trek that really tried a new path (and I really don't want to discuss whether or not it's a ripoff of Babylon 5, heh). It had long, complex story arcs that for the most part were well paid off. It had long romance arcs that got paid off, too-- you can see something building between Odo and Kira by the beginning of the second season, but they didn't get together till season six. Ditto with Bashir and Dax-- hints of that started in the very beginning, and in one sense it was never paid off, but in another it was, a long time later.
I think one problem a lot of people had with DS9 was that in season 4, they brought in Worf to bring up ratings, and the focus of the show necessarily shifted a bit. I like Worf (not my favorite Next Gen character, but I like him), so I didn't have a problem with it. At any rate, I felt that most of the original DS9 characters held their own just fine against him. And that's another thing I like better than Next Gen-- I like the characters and their relationships on DS9. It was the first Trek where they were allowed to get away from Roddenberry's decree that there shouldn't be serious conflict between characters. That led to the awesome feuding bromance between Odo and Quark, and the conflict-turned-bromance between O'Brien and Bashir. Not to mention the conflict between Kira and Sisko, and the larger conflict between religion and science (are the entities in the wormhole aliens, or the Prophets in their Temple?).
The only thing I don't like about DS9 is Avery Brooks. He has a beautiful deep voice, but I just do not like the way he delivers lines. He's way too pompous for my liking. I think he does okay as a Starfleet officer, but in more personal scenes (as in with friends, or with his son) Sisko just comes across as annoyingly stiff. He's my least favorite Starfleet captain, way behind Picard and Janeway. But Rene Auberjonois is amazing (Odo is my favorite DS9 character), and the rest of the cast does a great job, too. Overall, I think it's a very good show that's more consistent than Next Gen was.