Basic markup templates/frameworks can be helpful, if written well. They allow you to jump right into customization.
Design templates like from say templatemonster.com are great of you're having designers block and could do with some inspiration.
As far as a fully built site template...I usually prefer to build my own due to that fact that many templates intended for resale are slapped together very quickly, kinda like track housing. There are many factors that need to be looked at, quality of markup, css, and image optimization just to name a few. In the end it is often faster to build your own.