This mini-series is based on one of the most complex books I have read - dealing extensively with the inner demons of a man on an inexorable path to self-annihilation. I am happy to note that this mini-series does a superb job of translating the story from the book to film.
Yes, allowances have been taken - I don't think one could have made a film of this book without taking such allowances. Purists will object, and I'm sure each of us can find fault in some of the choices made, but these choices, by definition, are very subjective. As a whole, on its own the mini-series stand as a brilliant achievement - a great character study of a man's loss of his own character as he descends into dark abysses of continuous duplicity.
Peter Egan, a surprising choice for the role, does an outstanding job in the title role. A number of users have commented on how inappropriate he is for the role. I disagree, and again, I think the problem is the subjectivity of the subject. So much of the movie is based on his inner feelings and it is hard to convey that to the viewer. Some might prefer a more robust expression of his inner turmoil, but that does not really fit well with the character. I think, his more subtle approach is much more engaging and truer to what I imagined in the book - of course, others may have imagined differently and for them this may become a problem with his portrayal.
Overall an outstanding adaptation of the Le Carre book. Be forewarned that, just like the book, it is long and deliberately slow moving and may not be for everyone. Very little 'action' as such, but an exceptional character study of what makes a 'Perfect' Spy. There is a certain sadness which permeates the film, and becomes quite powerful at the end. Highly, highly recommended for those who prefer thoughtful, deliberately paced movies.
IMDB users have given this film an extremely high 8.8 (out of 10) rating as of January 2007.