ITK is an open-source software system to support the Visible Human Project. Currently under active development, ITK employs leading-edge segmentation and registration algorithms in two, three, and more dimensions.
ITK is a very big project for image processing. It provides image filters, means for image registration, and image segmentation. Considering ease of use you have to take into account that this is a library and not a fully fledged program. If you want some end-user program you might want to look up InsightApplications, Slicer 3D, Elastix, ITK tools, which are all based on ITK, or e.g, OpenCV, MIA for separate projects. I only give four stars, because to use ITK with C++ you have to become quite an expert in ITK style C++, SimpleITK will give you something more workable here, or you use other language bindings like python. Since ITK was initially conceived quite some time ago, ITK implements it own data containers that are generally not compatible with the STL interface. Also, ITK does not make use of C++11, and considering the list of compilers they plan to support until 2018 (Search Google for "ITK Release 4 Modern C++" for a timeline), it will take quite a while until these long awaited enhancements to C++ make it into ITK.