OpenCV is a very fast and mature one in C. The bad side is it is not object oriented, which is not one of your criterias I presume. Lots of functionality like connected component analysis are included.
JAI (Java Advanced Imaging) is fast and very mature and it is also object oriented. It may take a little bit to warm up with this library. But powerful and manageable.
If you are planning to make some academic research or prototyping go for OpenCV, if your aim is to develop a commercial or a large scale software, I recommend JAI.
UPDATE
OpenCV now has C++ and Python APIs. I have been using OpenCV C++ for 3 weeks now, the API is way better than C API. OpenCV is more stable, and community has grown. And there are lots of ready to use advanced algorithms in OpenCV. It is worth considering.
