Having used oscommerce, zen cart and more recently prestashop I would say prestashop is probably one of the easier solutions for building a relatively simple, attractive looking site. My main site is zen cart but I'm considering changing to prestashop. Unless you are an experienced designer / developer then I'd stay clear of oscommerce and go for zen cart or presta. Zen Cart is in a way a simpler version of oscommerce. Prestashop just looks more tidy and less like an "out of the box" solution to the untrained eye than os and zen. Some of the free templates are pretty tidy, and with the odd change to a few images it's relatively easy to customize the feel. Function is another story. As stated before there's limited documentation and community with presta - which i'm sure will grow steadily over the next year or so, which might make a beginer go with another solution.
