Some people like to tell you that Deep Space 9 is the best of all the Star Trek shows, because it stresses character development and continuity, and features a more complex background and ongoing plots. In some ways this makes it more satisfying, but in many ways the show fails entirely.
The series starts out as a soap opera on a space station, with two entire seasons of generic science fiction stories balanced with banal subplots about the characters. The characters are a good bunch, and most of the actors are decent, but I think the writers tried too hard to make them "normal". By "normal" they actually mean "ordinary and tedious".
At the end of Season Two we are introduced to the Dominion, who hang around menacingly for a while before finally going to war with the good guys in Season Five. This is the main "story arc" of the show, but it only takes up a fraction of the entire series. We still get lame stand-alone episodes, heroes still get stranded on weird planets for forty-five minutes, and there's an awful lot of low-brow comedy featuring the greedy, goofy Ferengi. A lot of episodes are merely dull, and some are unwatchable.
The Dominion, DS9's main villains, are bent on galactic domination for the convenient reason that, well, they just don't like anyone. The entire war is presented with a naive lack of moral complexity and imagination. Impressively pyrotechnic space battles appear with great frequency from Season Three onwards, but these are carried out in ludicrously simplistic ways, such as two huge fleets of super-advanced starships flying right at each other and blasting away. The writers of DS9 (including the talented Ronald D. Moore, later of "Battlestar Galactica" fame) spiced up their monotonous show by starting a war, but at heart it is still a pedantic soap.
DS9 remains a very frustrating experience. The continuous story is too flat and obvious to be really gripping, and the characters never truly develop in interesting ways. "Babylon 5" and "Battlestar Galactica" both fulfilled the promise made by DS9, and did everything much better. For Star Trek, stick with the original and the Next Generation.