I used Photoshop CS5 for this, and a Wacom Bamboo tablet. Set the brush tool to pressure sensitivity and start off with basic blocks of black and white to get some shape in, and slowly build out the form, keeping it rough. You then go along and add detail and sharpen things up. At the very end, add a layer and set it to "colour" and brush some colour over your black and white image to get that result. It's all about starting rough and slowly adding detail as you go along. Don't try to get things perfect on your first go, because you'll fail hard
Thanks for the comments guys, much appreciated