I didn't really notice a decline in the story until the Movie Arc and the decline is frustrating because it feels unnecessary. Essentially throughout the story narration is provided either by Aqua or Ruby. It keeps you focused on what's happening, explains how the various industries work, and keeps you routed in what's going on and what the characters are thinking, why they're thinking it, and what their plans are.
In the Movie Arc you get no narration from either person. Shit just happens and it is often ignored for dramatic effect and you don't actually see how your favorite characters respond to it. This doubles as frustrating for people because a lot of big reveals happen in here. The biggest being that Aqua and Ruby finally realize who they were in their past lives. Its after this that Ruby makes a very controversial move on Aqua and kisses him which got the anime labelled as an Incest anime because not only did this happen, you don't actually get any of the character's PoV on it, which is only made weirder when like the chapter before the anime was crucifying pedophiles and loli enjoyers as sex offenders.
However at the very end of that Arc you do eventually get Aqua's PoV as a former doctor and he sort of in one chapter sums a lot of shit up. He's not okay with Ruby's behavior, he understands that she views her current life as an extension of her past life and that she is the same person where as Aqua views himself as having been a doctor, died, and is now a completely different person with completely different circumstances to the point that he even considers his past life something that will fade with time and be nothing more than a nagging instinct he had when he was younger.