My views on Eureka Seven AO's ending
On the one hand, AO did not have a bad ending. It gave us good answers to our questions, it was well made, it had a bunch of quite nice action sequences (easily up to or exceeding the standards of the show to date, which have not been low), it gave Ao himself several good scenes to show various aspects of his growth, and it wrapped things up in a satisfactory way.
The problem is that in the process of doing this, the show took the entire cast of interesting, complex characters that we'd become emotionally invested in over the course of the story and reduced them to bystanders and spear carriers. None of them had their stories and themes resolved, none of them were given endings the way Ao was; they were all just ignored and wiped away. Naru was particularly badly done by (partly because her storyline raised some of the most interesting questions and themes of the show).
In effect the ending rewrites what the entire show was about. If it was actually about all of those character conflicts and themes that we thought we saw in the show, the writers dropped the ball at the end. My standard use of Occam's Razor says that instead, I was reading all of this depth into the show when the writers didn't deliberately put it there.
This has the unfortunate effect of reducing the whole show in my eyes. First, the whole thing leaves a bad taste in my mouth. I liked those characters and it annoys me to see them treated so shabbily by the ending. Second, the ending kind of turns Eureka Seven AO into a show about spectacle and Ao and that's not really enough; Ao himself is not a strong enough character to carry the show alone.
(In retrospect I wish that the other characters had known what was at stake if the Quartz Gun was fired again and had told Ao to go ahead anyways. That would have made it their conscious sacrifice as well as Ao's. But see above about reading things into the series that probably weren't actually there intentionally.)
Liked: I don't regret watching it. In the end it was still a good series; it had good production values, various moments of awesomeness, and good characters and character interactions. It was just let down by the ending.
Rewatch: No, and I'd actively avoid one. I don't think I could rewatch it just for the spectacle and for Ao's character development, partly because the other characters are such an integral part of the show right up until the last two episodes change that almost completely.
(This is kind of an elaboration of my tweet about this.)