Firstly, let me say that the movie, Watership Down, is my favourite movie of all time. So when I found out they were doing a series of the book I was delighted.
Aimed more at children than the original movie, the series kept closer to the book in the order that events took place (some were swapped around in the movie as well as other small details.) My guess is that whoever had the idea for the series wanted to relive the adventure of the book completely and in detail.
There are a whole host of famous voices to listen out for which makes the series even more fun, plus the characters are well animated and the background scenery is well drawn.
My only little niggle, which is a silly one I suppose is that the rabbits sometimes didn't move right, they'd walk like dogs (Bigwig in particular, who they gave a mane to instead of his trademark tuft of head fur) and after seeing the realness of the film, this kind of spoilt the series for me. However the rabbits weren't too toony or stylised and the movement thing was the only problem I had with the show.
So I treat this as a different interpretation of the book and a very well executed one!