***Some Spoilers *** 
 Finished it tonight. I really liked it through the ending (and the epilogues) and felt it read really well. There was enough mystery left in (the green card men) to retain the magic, but enough explanation to satisfy me. I liked the character development, particularly the romance, and the regularly occurring "cliff-hanger" events throughout. I could easily see this being made into a mini-series as the stages of the book make for several two hour installments, each complete unto themselves. 
 The twist on the time travel theme of always arriving at the same time, and each trip undoes the previous trip gave it a sort of Groundhog Day feel, though Jake/George ages normally. Also, the fear of making huge changes via the butterfly effect is well done in the last journey, as every little action becomes surrounded in paranoia, even to the point of worrying about buying a coffee. I started to see Jake/George in danger of losing his sanity much as the Green Card Man had if he stays much longer. 
 The only problem I had was that part of it was oddly familiar. The post-assassination attempt tornado and earthquake in particular reminded me of a short movie I saw 20 years ago in which a time traveler (a historian performing research) saves Kennedy and unleashes a series of natural disasters as time attempts to balance itself out. 
 All in all it was a really good read. I'll probably check out some other books by King after this since the only other thing I've read is Carrie and that was back in 1978 when I was 13 years old - I don't think I'll wait another 33 years before reading another.