Is /dev/random strong or just good?

Guy Rutenberg have written a little C++ class that provides random numbers on *nix platforms and he has published it on his Blog under a MIT license. The class used /dev/random and /dev/urandom to read random numbers provided by the kernel. He claims that the provided numbers are strong, even cryptographically so. I once read that "self-made" cryptography should always be avoided. This case, however, is a mix between a self-made and a ready-made, since the kernel provided random numbers are used. So, the question on security boils down to a simple question:

Is /dev/random cryptographically strong or just good?

I would like to have all the links you can provide on the matter. :-)