Well, I finished "The Sentinel" last night, and must admit I was quite disappointed in it (as I see from other comments, many other Reacher fans were as well). Over the last 2-3 years I've read Lee Child's other 24 Reacher books, and this one would rank at or near the bottom of the list.
The writing style is noticeably different (not that there's anything wrong with that; I guess it's to be expected when there are two authors). For example this book makes common use of short, choppy 2-3 word sentences, but for what effect? Also, the plot, characters and action just don't reach the level of those in earlier Reacher novels. I doubt that I would want to read any future Reacher books from now on.