Great series. To be honest, though, the trouble it had was the same trouble with adapting anything famous. To work on the screen, it has to be tweaked, and then you get the book geeks complaining. There was a session on UK's Channel 4 where they had some 16-year-old complaining that it was all crap bcos the cat room set was the wrong colour!
Let's be honest, the trilogy isn't actually that good - Peake does some very good set-pieces, but all the characters are so caricatured that you never really care what happens to them, and the third book is such ridiculously pretentious 60's rubbish that it's not even worth talking about, so it's just as well they didn't try to include that. In a piece about the series, one of the actors (I think it was the guy who plays the older Titus) said that the problem with all the characters is that they're all written as monsters, so getting any depth of humanity in them was difficult - in other words, they had to put some depth on the 2-dimensional characters otherwise it'd never work.
Frankly, Gormenghast is a book that nowadays would have gone through a few bouts of editting and come out much better. Lord of the Rings is another victim, although Tolkien is orders of magnitude better as a writer so he gets away with it - even so, there's sections where you easily lose a page or two in the Return of the King without really noticing.