This show would be my vote for most underrated anime show. Based on both votes and popularity, it is at the bottom on my "Best Of" list. In some ways its understandable and in other ways its not.
I'll start with it's obvious issues that I also second. A lot of its sci fi concepts don't get fully fleshed out, are half baked, and don't make a major impact to the plot. There are typical dumb anime moments seen in most shows that are cringe worthy. Some plot threads go nowhere and should've been developed more.
Now here is where I think despite all that, It has plenty good to offer and made my watching experience overall positive.
It has above average hand drawn animation that I miss in current anime. The action moments have some style and enhance the scenes. Great orchestral score that I still listen to. Characters you are interested in. Fast pace with constantly changing settings and plots.
The thing that won me over was the finale. Throughout the show it threw out unexplained sci fi concepts and plots that at first are head scratching. Then in the last two episodes, without spoiling, created twists and revelations that were both riveting and unpredictable. Its one of the few times my jaw dropped at a scene solely based on what was happening story wise. It catapulted this show from a slightly above average show to one of my favorites. Even to this day few animes I've watched have reached that emotional height.
I still look back on this show with fond memories of it even several years later. I don't look back on this show as some masterpiece or visual triumph. However, I remember it as one of the first sci-fis that got me both interested in the genre but also anime in general. For A lot of animes I first watched that I at the time enjoyed, I have changed my opinion on them significantly and in some cases outright dislike them now. Not for this show.
With all that said, I still understand the frustrations people had and I can see how the negatives will be too much to ignore. So please take my opinion with a grain of salt.