I finished Only the Open (Amazon link) less than 24 hours after I started reading it, and enjoyed it a great deal. The cast is huge and deftly-drawn, with lots of distinctive personalities, traits, and quirks to make even minor characters endearing. I loved some of the little touches, like the two enslaved Christians who debated religion to pass the time.
This is my favorite book in the series since Even the Wingless. I like the way the Chatcaavan Empire has become fleshed out as a more real and varied place than Wingless suggests, and that while the author still shows the casual cruelty of the culture, there's also Chatcaavans doing ordinary things and taking pleasure in pedestrian acts. In general, I think this worked very well: the point being that even monsters can be banal or kind. At certain points, they struck me as too flexible, too willing to accept the possibility that they were wrong, which made some solutions too facile.
But overall, I like the way the scope and variety of individual natures was portrayed, and the way experience of the setting varied depending on the character's place in it.
There's a lot of violence, torture, rape, and abuse in Only the Open (and a little bit of consensual sex). Not as much as with Wingless, but more than in Some Things Transcend or Amulet Rampant. Also lots of action and clever planning, some of which is amusing in its outside-the-action/adventure-box quality. It's the fourth in what will probably be a six book series, so it relies heavily on elements established in the previous books. It has a satisfying wrap-up of the central conflict for this book, but there are many more unresolved conflicts and some major characters are left in serious jeopardy at the end. I have faith Micah won't leave her readers dangling for too long before the next installment, however. I am rating this one a 9,