I like it as well.  My only comment is that it looks a bit more like a cloud than a brain because the general shape is off.  But I study neuroscience, so I'm a bit biased.  Here's an example of stripped down model of a human brain: http://www.joelertola.com/tutorials/brain/gifs/Brain.gif

Your colors remind me a bit of this image: http://www.zeitnews.org/images/stories/brain-763982-1.jpg.

I like that you refined the shape and lines very well.

But it of course lacks volume = shading.
The shading tones you used do no good, since they are almost invisible.

Actually I like it. Shadows and lights are made well and simple colouring is nice.