While the the best of them offer surprising amounts of flexibility, they also impose stringent enough restrictions to page design that you shouldn't be able to create a really bad looking site using one of these services. Typically you can get a Mysite.servicename.com style-url with no commerce abilities for free from one of these services; you have to pay extra for a better URL and the ability to sell. One issue to consider is that if you eventually outgrow one of these services, it can be hard to export your site to a full scale advanced web hosting like Dreamhost or Hostgator. If you know that's where you are eventually going, it may be better to skip the sitebuilder step.
These services can host your content on their servers free of charge, but in exchange for that zero cost, your online destination will have a less-than-elegant domain, such as jeffreylwilson.tumblr.com. That might be fine for a personal blog, but it will look too low-rent for a business that wants people to trust it enough to pay for whatever it's selling.