For me, this is the low point of Deep Space Nine's first season, the one episode out of several outstanding efforts that seemed to try its hardest not to be relevant in any way.
(I should mention that relevance and quality are not synonymous; technically "The Passenger" is competent, but there's just no reason to care about it.)
It's one of those Star Trek episodes that see a main character possessed by some evil mind, and on that score I would traditionally rate at least "franchise" as a reason to watch, but my standards for Deep Space Nine are a little different, especially being from the first season, especially being a series that went out of its way to say it was going to be different. Sure, you can watch this one if you're simply a fan of the franchise, but there's one glaring element that helps get in your way.
It features Dr. Bashir, the young Starfleet graduate hungry for some field experience. Except the episode could have been about any of the characters, and Bashir is simply too wide-eyed at this point for something like this to have any meaning to him. He was supposed to be the guy that reality slaps in the face. This may be Star Trek, and a Star Trek staple, but it's about as far from reality as you can get. Deep Space Nine, put simply, was supposed to avoid exactly this kind of episode.
So you should probably avoid it, too.
I'm doing a slight edit to this one. It's worth noting that Odo has an intriguing B-story involving a possible Starfleet replacement and/or rival who counts as one of the show's first recurring characters. It's not as memorable as other Odo developments from the series, but it's certainly noteworthy. I've upgraded the episode's rating accordingly.
franchise * series * essential * character
Memory Alpha summary.