This movie was released in 1991, ten years after the Urusei Yatsura debuted on Japanese TV. I guess for this reason, Itsudatte My Darling was a feature-length film rather than going straight to video like most of the other UY stories made after the TV series ended. As stated by other users here, this movie's unrelated to the fifth movie, and is a side story. Overall, I felt the movie wasn't bad, but it wasn't anything to write home about, either. The animation was done by Madhouse this time around, but don't expect the movie to look like one of Taro Rin's works. TAKAHASHI Kumiko did the character designs, and I was a bit bugged at her design for Lum, who looked less like an Oni and more like an elf. As for the writing, the story seems to focus much more on Lupica & Rio than Lum & Ataru. What I really disapproved of was the fact that Ataru is reduced to just a mere stereotype of himself, lusting after Lupica like an oversexed puppy, showing none of the devotion to Lum that makes him endearing.
This movie's only for die-hard UY fans who want to complete their collection, newcomers to the series are better off watching the first film instead.
Review by akira625 from the Internet Movie Database.