With any spinoff series there's a temptation to bring in characters the audience already knows, both because they require less exposition and because they can bring attention and ratings to the new series. Unfortunately, this sort of crossover isn't without its hazards: when the guest stars are used in a way where they overstep their supporting roles and become more central to the story than the regulars, there's a problem -- especially for a series that is still attempting to establish its own characters.
The DS9 pilot serves as a blueprint for how to avoid this trap. "Q-Less", on the other hand, is exhibit A for how to fall right into it. Vash brings a MacGuffin to the station, and Q chases her around while the DS9 regulars step into background roles.
As a Vash episode, it's alright. Jennifer Hetrick knows the character and is a dependable actress, though she's given nothing all that interesting to do here (the episode doesn't have much story to it.) As a Q episode, it's not so good: as John de Lancie himself once said, "Q is best used when he deals with large philosophical issues. And skirt-chasing just isn't one of them."
As a Deep Space Nine episode, it's watchable, moving at a nice clip, but ultimately unfulfilling which is probably why Vash and Q would never appear on the show again. (So long, Vash. enjoy the Tylenol commercials!)