Just saw it last night, and it was excellent. From an accuracy perspective, yes if you are looking, the bigger ships (the destroyers and mine-sweepers) were very obviously modern, but given that I think there are only a few floating museum ships from the period, much less active functioning ships, its necessary. That they got the actual MTB that Admiral Wake-Walker used at Dunkirk is remarkable.
It was absolutely thrilling to see flying Spitfires used in action, as well as the Buchons filling in for ME109s. They apparently used large drones for the bigger aircraft and the crash sequences. The CGI, if it was used, was totally unobtrusive. Here's a link for info on the filming
http://www.warbirdsnews.com/warbirds-news/dunkirk.html
The movie itself was remarkable. The tension that was built through the film was incredible and I thought the performances were great, as was the attention to historical accuracy.
Now if someone would just remake Sink the Bismark!