I only recently got into Ultraman with Trigger, but before that, this movie (and the sequel series Nexus) were the only Ultraman things I could get into. Even without the series, it's a great standalone movie.

If you watch it by itself, you do miss out on a neat little trick the Nexus series pulls off. The series revolves around mysteries, and the movie came out during the series to, if not solve, make those mysteries deeper. It was kind of a fun idea, even if it didn't work out commercially.