The fact that I read all three books in this trilogy in less than two months illustrates its quality. Great characters, lots of actions, an obvious fascination with spaceships (I'm tempted to get back into Eve-online after reading it), and most importantly - an excellent story.
Abaddon's Gate ties off the trilogy perfectly. I was left wanting more, which is good, but I also felt satisfied with the ending.
The writers have a knack for creating characters that seem very real and human. I found this made the book very emotional at points, probably a bonus for what is intended to be a space opera! (I don't recall feeling particularly emotional during any of the Star Wars movies, apart from being very excited!).
One character that isn't obvious at first, is gravity. By the end of Abaddon's Gate I had a much deeper insight into gravity and how it would feel to live on a planet or moon without much. It's a concept or 'character' I haven't seen used in any other sci fi universe, and it's done incredibly well.
The other thing I really like about Abaddon's Gate, and the trilogy as a whole, is it's easier to imagine that it might exist in the future. That is, I don't have to suspend my disbelief as much compared to say, Star Trek. This makes the world and characters even more believable.
Lastly...the book is just fun to read. I read an interview with the authors that said this was all they really wanted. They definitely succeeded for me. Part of what makes it fun is the action. The action sequences easily bring to mind a sci fi action film. I can imagine this would be relatively straightforward to convert into a screenplay...and hopefully one day it is (although I'm guessing the lack of gravity would make it expensive to produce :-)