Love it!
If you want to play with state-of-the-art machine learning software, this is not for you.
But if you want a clean implementation of neural networks in C that has a GPL license and no non-free components, this is a good start.
I have used FANN from Perl, amazing library. I'm still glad there is some AI software under GNU's belt and the source code for "gneural networks" is pure C and way easier to follow (for now)
It doesn't have quite the same feature set, but I wrote a simple, and I think "clean" neural net library for Python but with a pure C engine under the hood - could be pretty easily adapted to use directly from C.