Quick take: K:7S R:B gives us the origin story of the red and blue kings in a reasonably paced and well drawn anime.
If you liked the original series K, you'll like the first movie in K:7S. The story line is decent, but lacks much of the charm and mystery the original series provided. The story isn't bad by any means, but also lacks quite a bit of depth to make it any more than fair.
Art is good and on par with the original series, much like sound. Much of the great music is back, but there are also new tracks which I felt didn't live up to the original. Voice acting (Japanese) is good, but for the first time, I have to say I preferred the dub voice acting from the original series in my opinion.
Characters are good, but again lacks the depth the original series offers.
Overall, I enjoyed K:7S R:B, but don't think it's anything better than a good anime.