I haven't seen the anime yet, but I went to see the actual film yesterday, and I'm relieved to say that my fears were completely unfounded. It was excellent. Maybe not quite as good as the original but not far off, and a worthy sequel.
My one complaint was the lack of an interval. Once you take into account the pre-film ads and trailers, the viewing time was over three hours. My bladder struggles with that sort of timescale these days.