Do you want to use it build a site for yourself or to offer building flash sites as a service to clients? I would google wix.com reviews and base my decision on that. I haven't used it so I have no opinion. Perhaps try creating a free page for yourself and see how easy or hard it is.
Free website builders often come with 'free advertisements' built into them to PAY for the site builder; I would definitely check the fine print on that before beginning! I'm not familiar with Wix so can't tell you if that's part of their package or not.
When I started out, I paid $5/mth for GoDaddy's hosting and that included their Website Tonight software without ad support. It was really easy to use and customize.