Well, the OpenWorm project [1] estimates to be about 20-30% into simulating the 302 neurons and 95 muscle cells of Caenorhabditis elegans. The human brain has over 86000000000 neurons (about 300 million times more than the worm). I'd say we're about 0% into simulating (thus understanding) that. Mind you previous generation computers (32-bit) couldn't even count that high.
[1] https://en.wikipedia.org/wiki/OpenWorm