My son is nearly 6, and has generally found all movies scary, all the time. He saw his first in-theatre movie a few months ago (at his request) and he stated he would not go back, ever. (It involved his favourite building blocks.)
However, he watched the Bears previews with me, and he asked to see the movie, and he really enjoyed it- even said he would go back with his Dad. Somehow, the live action with the bears was much easier for him to manage than anything animated. He was a bit worried at times, but not to the point that he was demanding to leave.
As an adult, I found it enjoyable, though definitely a bit Disney-fied. I was surprised that they so openly showed the cubs nursing on the one hand, and then the fish being eaten alive the next. The filming is spectacular, and many of the scenes are just amazing to watch.
I found the narration a bit trite, but it suited the almost 6-year-old sense of humour -he's still repeating lines almost a week later and laughing madly so clearly he enjoyed it.
Overall - a big hit for us, and it opened the door to more movies in our future - though perhaps just documentaries for now! :)