One could write all the ways that this film lacks in structure, as well as the issues from inexperience and lack of creative freedom. Alas, this is a movie that I find strangely difficult to hate.
The largest strength of this film is that it did not have to live in the shadow of its landmark successor and the best film of the series, Beautiful Dreamer. All that came out after it had to be the next BD, and therefore strayed even further away from the series (with the exception of Final Chapter, as it is an adaptation of the original manga). It is ironic to think that Oshii Mamoru, the one who directed the film that sparked a minor controversy over disrespecting the source material, has directed the film that is most like Urusei Yatsura.
If one was asked to imagine a spinoff movie of Urusei Yatsura without being aware of the existing films, this would be the resulting imagination. While reportedly not satisfied with his lack of control and a having general disdain for an endless series that never progresses, Oshii still did a generally competent job with constructing its plot arcs and comedy. Though perhaps not true to his vision for the film, the way fanservice is handled is a fine line between a lack thereof and intrusiveness, and therefore feels very much like an extension of the series.
The almost vacant ending rings differently from the weaker endings of Remember my Love and Lum the Forever, using a rather powerful and telling sequence to bring the movie to an end in an anticlimax. Unconventional developments such as this give the film a unique charm that is distinct from any other film of the series.
Those who enjoyed the Urusei Yatsura series will most definitely enjoy this film, while it remains an interesting film to analyze as an early work of a great director.