It's big enough for me to make decent money and growing all the time. If you use Shopify's billing system they take 20%.
I've written the popular PHP client adapter for the Shopify API (http://wiki.shopify.com/Making_A_Shopify_App#.E2.80.9CI.E2.8...). If you need any help with getting started with Shopify, feel free to get in touch with me.