I thought Niffenegger presented a superbly novel approach to the "time travel" aspects of the book and I felt that, for the most part, the logic of the time travel premise holds up well. There are minor issues which others have mentioned but given the challenges of writing about time travel I think the author did about as well as can possibly be expected.
My main complaints are with regard to the overly long digressions involving wine & cuisine, art, pregnancy issues ect. They smacked of filler rather than necessary information which was in any way pertinent to the story. A little of this would be OK but Niffenegger often dragged these things out to the point where I wanted to say, "Enough already! Get on with the story." This having been said, I would (and have) still recommend the book. It was, overall, a good story and a very original take on the time travel theme.
“I just got out of the hospital. I was in a speed reading accident. I hit a book mark and flew across the room.”
― Steven Wright