I have shops on three Print On-Demand sizes - CafePress, Zazzle, and (just recently) Skreened, which I've heard great things about.
So far, I've never sold a product on CafePress - maybe the marketplace is just too crowded. I sell products on Zazzle almost daily, which I attribute to the customization options they give you (my fitness business card templates are my top seller by far). And I've just put up my Skreened store, and wound up selling one product only one day after it was online.
After the tedium of setting up shops on CafePress and Zazzle (not their faults - they both offer a wide array of products that require lots of size and positioning tweaks, and different images are usually required for their dark clothing) I wasn't ready to set up another shop, but Skreened's ethics (no sweatshop labor, environmentally-friendly shirts) won me over. Plus, they only offer light colored shirts and a canvas bags, so the shop took just a couple hours to get up (even with over 40 designs) due to the limited product categories. You can't even customize the look of your shop - who would have thought that was a benefit?
My Skreened Shop