From the first (teaser) scene, we can see the warning sign: not enough story filling the scene. And then we have to wonder if there wasn't enough story to fill the episode, since O'Brien has to narrate it for us. Sure enough, there isn't. So why does everyone else rate this so high?
The premise is great, but the details don't hold up. Why does Keiko make his favorite food, but not eat any herself? Answer: to satisfy the story mechanics and no other reason. Why does O'Brien trust people and get disappointed, when he already suspects a conspiracy? Same answer. Why didn't they just thrown him in the brig until the situation was resolved? See previous answer. And then the denouement happens far too easily (a common problem in TNG-onward Trek) -- even without the final line about Keiko, which doesn't bear up to examination.
A much stronger episode would have started with the last-scene confrontation, then flashed back to the story's beginning. Keiko's behavior should have been something more subtle than straight-up acting suspicious. And Odo should not have been thrown under the plot-device bus.
Which is all too bad. O'Brien and Odo are two of our favorite characters on the show, and we are quite familiar with the Phil-Dickian storyline attempted here -- anyone who's seen Total Recall will see the similarities. But the denouement only tells us what happened, not why or how or with any sense of a larger picture. So the end result is a great portrayal of a mood and perspective which simply doesn't connect the dots to make sense. Looks great, and anyone who saw a few minutes out of the middle would definitely find themselves hooked. I just wish they'd finished the job right.