I usually do not like summary movies: they are mostly useless and poorly done. You should always see the original series, and it is the case here as well.
However, I must admit this is probably the best summary movie I have seen. Kyoto Animation wisely focused on Asuka, inserted all great moments of the original serie, and even added some extra scenes (a few minutes). They also proposed the final concert in an extended way, which I found magnificent (and missing in the original serie).