This Klingon episode featuring Worf and Martok is most notable for being the last appearance of Kor (John Colicos), Star Trek's first Klingon, whose roots go back to the original series episode "Errand of Mercy". (He also appears in the animated series, voiced by James Doohan). Taking place largely aboard a Klingon Bird of Prey, the substance of "Once More" is hardly anything new for Klingons; we get the petty bickering, mess hall insults, a battle, and the required variation of "a good day to die". But working its way through all the old clichs, the plot is a thing of beauty that's well written by Ron Moore and nicely performed by the cast.
Like "Soldiers of the Empire", the point of the episode is to set up an underdog for an elusive victory. But this time it's more personal, because it's not about a ship, it's about Kor. John Colicos, in his third DS9 episode, does a magnificent job closing the door on the famous Dahar Master before his own passing in March of 2000. Meanwhile, J. G. Hertzler nearly steals the show, playing Martok with such gravitas you'd swear he's the one who once went toe to toe with Captain Kirk. Like in "Soldiers", Martok serves as a bit of an antagonist, but whereas there his motivation there is unclear, here he gets a well written back story that allows us to understand where he's coming from.
But it's Neil Vipond as Derok, Martok's servant, who proves the biggest surprise. Vipond doesn't have a lot of screentime early, and it's easy to dismiss him as unimportant, but he does the most with the least, setting up his importance later with a subtlety that's easy to miss the first time around.
Throw in Michael Dorn, bringing his umpteen years of experience as Worf to the table, and you get a Klingon episode that, in the spirit of Henry V, goes once more into the breach and savors the fruit of victory.