Olav seems to give a fair summary at the following link.
http://computer-programming-forum.co...b40f9d52e9.htm

I assume it is not necessarily faster than well written code that accesses GDI/GDI+ and that take advantage of the "tricks of the trade" to get high speed access to pixels.
But the point is that it provides better and wider access to these capabilities, so the end user can take advantage of a larger base of 2D drawing capabilities without having to extend GDI/GDI+ themselves to encapsulate them.
It sounds especially useful for VB6 users, and of course is geared for C from the start. Probably will require some interop considerations if you wanted to use it from VB.Net or C#. (Actually, a quick search shows a language bindings list here.

I guess that's all I have time to investigate for now.