I agree that this is one of the stronger romance episodes, to the extent that I think it's dignified enough to "frown at the clown" and demand a proper introduction
. Which is to say, despite demonstrating the usual implausibilities of romance-of-the-week, there's enough of a point to this one to elevate it out of the RomanceTrek box. Perhaps it's because Odo is new to sexual relationships, and a person's first such experience carries with it the sense of experimentation that lets us overlook the need for a partnership to develop at a sensible pace. It's more an "Odo gains life experience" plot than a "character suddenly falls in love" piece, which makes it more palatable. Perhaps too it's because the underworld/intelligence theme brings to mind genres in which sudden sexual liaisons seem less out of place; there's a actual backdrop to the relationship, even if it is a rather cliched one.
I agree entirely that this episode would've been more effective if Odo were still a solid, but I'm perfectly happy with what we got. Arissa's final line is great, I must say, and I'm not sure I "got it" the first time. "I'll never forget you" sounds like the usual fare from romance-of-the-week, a somewhat hokey attempt to demonstrate that this character we'll never see again was a True Love, but considering what Arissa does for a living, it's both tragically poignant and darkly amusing. It also manages, then, to carry a (probably unintentional) barb that challenges the notion of meaningful, never-to-be-regretted-or-forgotten whirlwind romances. It's a truly DS9 sort of RomanceTrek.