I just finished today, and I agree with a lot of what has been said here already.
I think I had built this book up so high in my head because I'd heard such rave reviews about it. Honestly, I had set the bar so high that it would've been difficult to reach it. Part of my problem was that I had recently read a book on a similar topic that I thoroughly enjoyed (
Replay by Ken Grimwood), so I spent a good part of the book comparing it to that.
I found it very difficult to follow all of the back and forth, the random mention of events that didn't seem to further the story at all, and as Nikki pointed out, the lack of emotional connection to the characters along with all the science mumbo jumbo was almost too much for me. I found myself skimming large sections.
Admittedly, it started out really interesting, but I was soon having to force myself to get back to it. It was seriously SO slow going until I hit about the 60% mark. I read from there through the end of the book pretty quickly.
The ending was mostly satisfying, I think. It wasn't the best ending of any novel that I'd ever read, but I felt that the author tied the story up nicely.
All in all, I agree with Brie that I liked the book, but I didn't love it. I don't regret reading it, but it won't be one that I ever feel the need to reread.
Now I'm off to read something fluffy that requires no thinking whatsoever. LOL