I ended up reading the entire Giver Quartet (The Giver, Gathering Blue, Messenger, Son) almost one right after the other and found that what I wanted to say for each book was so similar that it made more sense to just put all four books in the same review rather than repeat myself.
Each book focuses on a kid around the age of 12. All of them are just reaching the point where they suddenly are gaining more adult responsibilities, but also learning that the world they knew holds far more secrets than they had realized. The places they live in vary. Jonas' world in The Giver is seemingly perfect, as is Matty's in Messenger, while Kira's in Gathering Blue is definitely not. Claire's journey in Son takes her from Jonas' original home to other places. Regardless of how good or bad their worlds seem at the start though, all the protagonists learn that there are problems they weren't even aware of.
When I started the second book of the series, I wasn't really sure how it connected with the first. The setting and characters are so completely different, I was even sure if the books existed in the same world. The third book ties the two together though, and the fourth continues all their stories.
The books all leave many questions unanswered about the various societies, and there are plenty of parts that left me questioning how exactly things worked, but the world-building was never really the point of the series. The world-building exists more to work as a metaphor to get the desired message across about mankind. One of the metaphors is outright explained by the protagonist in the third book. Normally I get bothered by world-building that doesn't make sense, but I didn't mind that here, probably because the author doesn't try to explain how exactly things got to the point we see. It's more of a 'just go with it' kind of a story, and the books had my interest enough that I was perfectly willing to go with it.
My biggest problem with the books had to do with the endings. Each book goes along letting its story unfold until it reaches a point where things suddenly start happening (with sudden additions of weirdness for some of the books) to get the protagonists to that final conflict. And then it just ends. Every book left me feeling like it was missing another chapter or two because the endings just felt really abrupt. It always felt like the author wasn't entirely sure how to end everything, so a bunch of last minute things were thrown in out of nowhere before she got tired of writing and just stopped.
Overall however, I enjoyed the entire series, despite being underwhelmed by the endings. They all drew me in rather quickly and kept me interested.