Personally I would not use SharePoint Designer on a public internet site. While it might be comforting for people who are migrating from FrontPage because it has support for all of the FrontPage bots (requires FPSE on the hosting server to use them) its code tends to be more proprietary and works best with Internet Explorer. While it is possible to create websites with SharePoint Designer that work with all web browsers it is harder to do so than it is in Expression Web.
Like TexasWebDevelopers we use a variety of programs here which includes Dreamweaver, Expression Web, SharePoint Designer and even Visual Web Developer Expresss or Visual Studio.we use each for what it does best. SharePoint Designer is used on intranet and extranet sites only. We also work with a variety of technologies including ASP.NET, Classic ASP and PHP so that influences which one we use as well.
If you will not be working on a website hosted on SharePoint Server or Windows SharePoint Services I stay away from using SharePoint Designer but your mileage may vary.