This is a movie that I considered seeing at the theater when it came out but which, for one reason or another, I didn't. I happened to catch it recently on Netflix.
First let me say its a good move and I enjoyed it. Secondly let me say that people shouldn't take their history of Benchly Park, MI6's code breaking operation during World War Two, and the British work on code breaking from this film.
The film is a fictionalized account of the story of Alan Turing, the British mathematician who was a central figure in breaking German ciphers associated with the German use of a the military Enigma machine. The story, if taken at the 30,000 foot view, gives a rough approximation of what occured in that endeavor, but only if taken at that view.
That doesn't make The Immitation Game a bad movie by any measure, except perhaps as the straight historical one. Turing truly was central to the story of breaking the codes associated with Enigma and with the construction of the Bomba, a proto-computer that allowed the British to break an appreciable percentage of Engima coded messages. The portray of Turing's character seems to be on,. and the story is loosely, although I'd emphasize loosely, portrayed correctly albeit in a ficationlized and very simplified fashion. And that's the problem for a person who is historically minded.
An accurate story of these events would be supremely interesting, but quite complicated, and slightly disappointing. The effort involved by Turing, and others, was so supremely cerebral that it would frankly be extraordinarily difficult to depict in film. And the story of British code breaking is much more complicated than simply that of Enigma. The British broke, for example, another German cipher based on wire telegrammetry that didn't involve Turing and actually was nearly completely simply due to the mental deductions of the character who broke it. The use of Ultra, as the broken code information was called, was also highly complicated and not accurately portrayed in the film. Soviet penetration of Benchly Park was much more extensive than the film would allow and not at all winked at by MI6 as suggested.
In short, it is a much more complicated, and interesting, story than set out in this film.
One thing that should perhaps be mentioned is that it does seem, to the extent I'm familiar with it, that the film got the story of Turing's homosexuality correct. That has little to do with his code breaking, of course, but it is an element of his personality and that part of the story seems to have been done correctly, to the extent I'm aware of his personal story, which isn't all that much.
So, a decent film worth viewing, but don't take it as an in depth history of MI6 or Benchly Park.