The only reason they used Q, I like to believe, is to show a nasty streak they wouldn't dare do in TNG. It's as much a teaser of what Q could be as it is a crossover, which by this point wasn't needed and other characters would somehow manage to show up in later episodes...
The scene where Q snaps his fingers and gives Vash back her rash she acquired from the one planet they visited is something of a grizzly scene. Though Vash could have been any person, her return is a bit of direct continuity.But unlike the denisens of DS9, at least we felt for her as she's crawling around the Promenade in agony and looking like a puffy pustule mess and nobody seeing her gives a damn to even call security or infirmary or anyone. That's pretty start. What would Dr McCoy say if he were a fly on the wall and then realizing this isn't the mirror universe?
It all does get a bit heavyhanded by the end with the ridiculous boxing match, complete with campy mustache, where Sisko has to tell Q he's not Picard. DS9 already showed enough times this ain't TNG, even in this episode by using Q in a more grizzly way. Were there no interesting ways to let Q bow out of the episode than this pale imitation of Rocky DS-IX?
DS9 was finding its way, DS9 did use other characters as crossovers for far less reason and less convincingly. Q and Vash didn't need to be in the series, but ranking the crossover episodes, Q-Less (interesting title, especially if Q were never thought of being used, much less not used again) is arguably the best of the bunch.
If nothing else, at least the best of Q's traits (magical Jeannie/Samantha powers aside) are all distilled into Garak anyway and 7 years with him are too few.
IMHO I couldn't make this one the worst of DS9/S1. The one where Bashir's body gets taken over and it's so early in the series that Alexander's personality as possessed became a plank of wood for those few minutes. "The Passenger"? The other clunker (IMHO) was "Dramatis Personae", which was DS9's worst reimagining of a TNG episode that had the same plot points ("Sarek" where some telepathic influence causes the crew to go nuts), though this one has shades of "The Tholian Web" in terms of people going bonkers and trying to murder one another as well. Two good episodes they borrowed from but the end result, at least for me, just didn't work. But I'd rather this one be the worst of the lot than the best of the lot and that's inevitable for every season or series.