It's obviously easier to start with extending an existing theme, but I also find it limiting, in that you sometimes spend extra time writing CSS to override the existing styles to get the theme to do what you want. In the end, you are stuck with someone else's work.
Rather than start completely from scratch, consider using a base starter theme. I use _s (aka Underscores), but there are others (Bootstrap, Bones, Root, etc) You start all the necessary parts of a theme, but no styling at all. From there, you make it your own.