I checked, and many of the contributed modules that I use have not been ported to Drupal 6. This means that I cannot upgrade without losing functionality. Before beginning a new site, you'll need to consider which modules are available and make your decision based on that.

I am excited to start looking at Drupal 6, but my work load prohibits that I spend much time on it at the moment. Perhaps when I finish my current Drupal installation series I'll take a closer look at it.