Well, as a world war enthusiast among other things, I have read a lot about Enigma and its working. It is safe to say that had it not been for Enigma, Germans would have lost the war a great deal earlier and ironically it was due to Enigma that they lost the war, as they thought that its codes were unbreakable.
Robert Harris has definitely done his research well in the fields regarding Enigma and code breaking done at Bletchley Park during World War II, but I had a feeling that it just served as a backdrop for what could be essentially considered as a love story.
The book lacked excitement, which given the backdrop, should not have been the case.
So, if you have watched the 2001 movie of the same name
starring Dougray Scott and Kate Winslet, then you are loosing nothing by giving this book a pass.
If you want an alternative, you might want to check out another book called The Enigma by Michael Barak. It revolves around stealing Enigma from German hands and while it has some cardboard characters, I found it more memorable than this book.