When someone says "it will just be like a vacation" at the start of a story, you know it won't be a vacation. It's like saying "what can possibly go wrong?"😉
We see Kai Opaka as part of Vedek Bareil's vision. This was the first time we saw her after "Battle Lines". She would return for one more DS9 episode. (from MA).
DS9 will return to the Mirror Universe a few more times during its run (around once a season). Of the DS9 Mirror Universe episodes, I enjoyed this one the most.
I do like TOS" "Mirror, Mirror" and ENT's "In a Mirror, Darkly" episodes a bit more.