I was engrossed the entire time. I thought it looked beautiful, really stunning. I enjoyed the multiple timelines, thought that was brilliantly done. The sound editing was amazing, those rifle shots made me jump every time, even though I knew they were coming.
However, the music was too over the top for me in places, the high pitched violins etc. I did appreciate the clock ticking in the music at different tempos depending on which timeline was being presented, but Zimmer is just too much for me sometimes.
I also never got the impression that were actually 400,000 on the beach, visually. It looked like lots of soldiers but not as many as that. I understand that the beach was miles long so they were spread out, but I think the shear numbers of soldiers rescued, a miraculous feat, could have been portrayed better.
All in all, I thought it was very well done, and would see it again. Also, see it in the very biggest IMAX screen you can.