Not sure about serious critique, this really isn't my area, but I felt I had to write something as it's such a striking piece and caught my eye straight from the thumbnail.

I think you've pretty much done everything as I would want to have done it (had I the ability and patience). It's not overly detailed, has an abstract feel and the lines flow like a face soup. Probably the only two niggles I have with it are the way the beard seems oddly truncated below the chin along the dark vertical line, I'm having difficulty imagining it as a photo (if you see what I mean). The other small point is the green in the hair, for some reason (I can see what you were going for) it just doesn't sit well with me and looks out of place. I think the spectacle reflections alone should be a solid point of colour focus in the piece and the hair accents justa different shade of grey... but hey, who knows, you might have green haire

All in all I'm really taken with it and if anything it's inspired me to have a crack at it myself!

Well done.