I picked this up with some trepidation--partly because Dallas is my home town and partly because it's such a big fat book.
But it was good! Lots of food for thought on history and time and how time travel could work. The characters were all wonderful--deeply layered and complex. It was a page turner, though I still think that perhaps 100 pages or so could have been cut. King spends a lot of time setting the stage before we ever get to Dallas.
I'm no expert of the JFK assassination, but I do know a little bit more than the average bear (I spent a very memorable summer working at the Sixth Floor museum). It rang true, and that was what I was most concerned about.

Side note: I know 5 people named in the acknowledgments. One of them is my former boss (and she's still a good friend). Very strange to see their names in a book at the top of the best-seller list! In some ways that makes me like the book even more. I know King did his homework.