For some reason, the first time I saw this it did not have the gripping impact of "Whispers" or "Hard Time". But this time around, I felt pretty comfortable putting it in the same class.
This is one of those extremely densely written episodes - the kind that feel like they pack 60 or 90 minutes into a 45 minute space. For an O'Brien-centric episode, there's plenty of stuff tackled here - Klingon espionage, Romulan intrigue, Odo's detective skills, and of course the Kira-Odo romance.
And yet the man of the day stands out here. He knew how to play the tormented O'Brien perfectly (we got the first hints on TNG's "The Wounded"). At first the visions of the future seem inevitable. But then we get visions that turn out not to be. And finally, we get an unsettling pretzel of a time-twister; even O'Brien agrees that it doesn't (or might not make sense).
The ending's switcharoo disturbs O'Brien, and it disturbs O'Brien - who knows he's the wrong O'Brien for this time and place.
Trivia: this is the second episode with the dartboard, and the first one in which the dartboard arrives at Quark's. An amusing sequence in which Quark tries to throw darts ensues.