Watership Down was one of my favorite books of all time (this was before I discovered Ursula K Le Guin, however ;-) ). I was pleasantly surprised with the movie version. It was a cartoon, obviously, and omitted quite a bit of the (very long) story out of necessity, but still did a faithful job of representing the plot and characters in my opinion.
speaking of Ursula Leguin they made a terrible adaptation of Lathe of Heaven a few decades ago. sorry, I was surfing the writer's block answers and I saw your ref to LeGuin and felt the urge to comment.
Watership Down is one of my favorites as well. I don't think any movie could accurately cover the many concepts within the book. Few other stories describe the problems associated with political leadership as well as it does.