Tom Geller

I disagree with yaph, for one overwhelming reason: Security. There are enough Drupal 5 sites out there that online attackers would find it worth their time to create tools to break it. Now that D5 is no longer maintained, there will be no fixes for new cracks.

So even if your D5 site is running well, upgrade for security's sake, or it might not run well -- or at all -- for long.