Support among the services varies widely, from free WordPress.com account's only offering community support, to Jimdo's email-only service, to Wix's telephone-callback service—even for free accounts! Many of the site builders offer rich online support knowledge bases and FAQs, so there's a good chance you won't even need to contact the company. I test each service's support as part of the review process by asking how to connect a domain bought elsewhere to my site and how to sell digital downloads.
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.