I gave the first three books 4-stars (exciting but with thin characters) and half-way through A Darkling Plain was of a mind to give this 5-stars, to acknowledge the worldbuilding and story aspects that spanned all the books, even though it struggled with the same problems. (See what I did there, Philip? 'Even though it struggled' instead of 'even though it was struggling'.) The final third of the book cemented a begrudging 3-stars, it was just terrible.
Before I even talk about the story and characters, I have to mention the writing, or perhaps that should be the editing, or its lack of editing. Throughout the series, the writing was solid enough if nothing spectacular, and rarely distracting. For a while I even thought the quality had gone up in A Darkling Plain, but like the story, it seemed to collapse in the final third. Packed with action, yet filled with passive tense - these lines should be URGENT and TAUGHT but instead were weak and lazy.
"The Stalker's robes began to burn. Lightning was crawling across her calm, bronze face..."