I had high hopes for this book since it combines 2 of my favourite genres - timetravel and historical fiction. 
 The premise is awesome - a young english teacher discovers a portal to the past and is talked into going back in time to save JFK. 
 The execution of the premise, however, was not so great. I suspect King either rang Connie Willis or read some of her books when writing this, because this book too could have easily been half the length ( it takes until EIGHTY PERCENT of the way through the book to actually get to the assassination.) Just as in Willis's books there are many, many filler chapters where nothing at all happens, annoying repetition (and repetition) of key phrases (just replace Willis's "but this is time travel" with "but the past is obdurate") and the irritating way in which the long anticipated action is cut off just when it all starts to get interesting (of course his 'accident' involved memory loss -- cue even more annoying filler chapters where nothing at all happens). 
 It felt to me like King did a huge amount of research into life during the early 60s and used every opportunity to describe the food, the clothes, the cars and music and insert every slang word his research had uncovered. What was missing though was some emotional connection by the time travelling main character to these differences. This review by Sarah Kathryn pretty much sums it all up - there just wasn't enough depth to the historical setting. It's not enough to just say how things look, you have to make your character actually *feel* something about how it looks. 
 I do still think the premise is awesome. I love the idea of someone going back and changing a major event - the possibilities to be imagined are endless! King's version of an alternate future involves (view spoiler)[world ending earthquakes. Earthquakes?! (hide spoiler)] Yeah. That pretty much sums up everything that is wrong with this book.