I have tried writing much for it, but it is difficult to find a way to properly describe the charm of this film. There is the discordant atmosphere that feels almost unsettling, a surprisingly thoughtful handling of a more difficult material, a fantastic score that gives life to energetic and wonderfully animated sequences, and a powerful ending that acknowledges the fourth wall in a clever, unique way. As to not ruin one's enjoyment of this film, I will give simple guidelines that a viewer may find useful as opposed to an extensive review.
For those who have watched/read and loved Urusei Yatsura, Beautiful Dreamer is a film first and Urusei Yatsura second. While it has very much the same characters and they aren't understood particularly badly, the plot distances itself from the general atmosphere of the series quite a lot. If you enjoyed the more quirky story arcs with time travel and alternate universes, this is a film version of such arcs with improved depth and exploration of the themes.
For the fans of Oshii Mamoru and those who've watched Ghost in the Shell, the fact that they've been directed by the same director really shows. While a very early work in his career, many of the posthumanist themes discussed in his later films are apparent in Beautiful Dreamer. In contrast, it lacks the political tone of his future works, as well as missing the more esoteric and avantgarde style that he would adapt later. If you found his films difficult to watch but was interested in some of the ideas he developed, it's a good film to start with.
For those who are just looking for a good movie to watch, give it a go. While not a film for everyone, it by no means is difficult or esoteric. That could perhaps be the largest difference between this film and others that explore similar themes, and often to more depth; at the end of the day, Beautiful Dreamer is a bit of unserious entertainment that has a powerful spark of creativity. It's a little inexperienced, inconsistent and experimental, and that's maybe why we like it so much.