I think it's really good, actually, especially for a first time. The edges of the sharpied areas are rather clean (mostly, at least), and as far as I can tell, you haven't cross any outlines. Accurate work, overall, and pretty cool design, too.
As for criticism, the sharpie work on the helmet, mainly on the fin, doesn't seem to be as smooth as on the torso. There are, for example, some white spots visible on the edges of the fin, which increases the unclean look.
In addition, the sharpie paint itself looks uneven - meaning that some parts are darker than others - but I assume that's the case with all sharpies.