I finally saw this movie as an adult. As someone married to a British person, I'm trying to watch all the British children's movies that I missed growing up. Of all the British animated movies, this is one of the greatest works. Overall, they did a beautiful job on the landscape. It totally looks like a true British countryside. I also really like the song "Bright Eyes" written by Mike Batt and performed by Art Garfunkel.
I've also heard this film is controversial, because it does contain some grim imagery. I'll be honest, I would recommend exercising caution over showing it to children under 10 years old. There are a few scenes with animated blood, and I could see how the predators to the rabbits might give young children nightmares. I don't particularly see why it should be controversial, because there are many popular children's movies that don't shy away from death. In "Babe", when the sheep gets killed by the dogs, her wool is full of blood. Not to mention, in "James and the Giant Peach" his parents get eaten by a rhino, forcing him to live with his mean aunts.
Another impression of this movie is that it kind gives kids an early lesson about dictatorship, through how General Woundwort tries to hold the other rabbits under his power and torchers them if they try to escape. When Bigwig leads some of them out, it gives an example of being strong and rebellious.
Overall, this movie is intense, but I liked it. It might be too scary for some kids, but some would probably feel comfortable watching it. I feel like it's gifted in truth. It's important for kids to learn that nature isn't all sparkle and shine, so they can be prepared for the outside world.