I just finished the first season of Yu Yu Hakusho!
While I absolutely love the series so far, this non-canonical side "movie" is all filler and no killer. It plays like the show's least interesting episode, with an overly simplistic story and characters whose normally vibrant personalities are reduced to nothingness by a weak script.
It isn't entirely without merit; the animation is beautiful and there's some good comedy here and there. But that's about it.
Overall, this movie is watchable but far from great. Mostly, though, it's simply there. It exists, but adds nothing to the experience of watching the series. I should've just watched the next episode...