This project is the best library tool I have found for handling image metadata.
Very complete API, and the support is much better than even commercial applications (this is for you, Robin Mills). However, this project is free and open-source.
The only weaknesses I see are that it's written in C++, which makes for some technical and design constraints, and its GPLv2 license is somewhat restrictive.
Apart from these, here is from one happy user.