I just finished it today and I really enjoyed it. I wasn’t expecting to.  Under the Dome had some good parts, but one of the worst endings of his… and that’s saying A LOT.  I don’t even remember what Duma Key was about other than some vague details.  I am quite sure I am not alone in being extremely disappointed by the last few Dark Tower books.  So even though the short stories and novellas still are mostly good, novels haven’t done it for me in a long time.
When I got it and saw it was 800+ pgs and I only had 2 weeks to read it (no renewals), I was filled with more dread.  I finished it in less than 72 hours, which is no record for me, but still nearly 300 pages a day and pretty respectable, as I did many things other than reading.  I really didn’t like putting it down and I’ve been kind of sad since I finished it, a phenomenon that hasn’t happened with SK in a while.  I don’t know if the book will keep well over time.  If I reread it in a few years, I might think it’s crap.  But right now I like it and it feels nice.
I don’t think the 50s and 60s were too rose-colored.  If it wasn’t portrayed as appealing at all, it would make George/Jake’s conflicts seem stupid.  I am the same age-ish as George, so it was kind of annoying sometimes when he’d know some esoteric bit of trivia from history class (not his best subject, he’d say and explicitly say it wasn’t from Al’s notes), but then sometimes was completely ignorant of things that seem pretty obvious to me.
I do think the book could have used some trimming, but I think SK is beyond editing now.  It wasn’t tedious, but it could have saved some trees.
And he can explain it by the syphilis or whatever, but the whole crazy bookie was too much for me.  I’m not saying a bookie would have hugged George and given him a lollipop, but it was voodoo-shark level of determination for what really could have been a lucky bet.  If George had done something more blatant or if he had run into the same guy in TX, it woudl have been more believable.  George/Jake pointed it out himself, but stocks would have been safer.  Or Vegas, as mentioned upthread.
I think Al was a dumbass for waiting so long to get Jake’s help.  I think his waiting was the past being obdurate through him, making him a dumbass.
The saved-JFK timeline was so messed up not just because of the changes Jake/George had made in that one trip, but because of all the changes made by all their collective trips. This was said by the card guy at the end.  So it makes me wonder that if Al and Jake had only made a couple trips each, would the saved-JFK timeline ended up so badly?  It seems like the implied moral was “If you find a door to the past and you want to save the president, don’t waste all your visits on buying meat for cheap.”  (And why wouldn’t Al charge close to market prices and keep the difference as a profit, rather than having very low sales because people were afraid of his cooking because it was so inexpensive?)