Cue the stirring music and let loose the doves, I've upgraded this website to Drupal 6. It wasn't too much of a pain, but it helps that there's not a lot of nested whirling gears going on here -- this is a pretty straightforward, contrib module light site (at least for now).
So why? Well, if you don't keep up, you get left behind. Regular upgrading, no matter the pain, is actually what you want in software. No upgrades = dead product. And with open source software, it usually means a better product. I was in plenty of meetings at AOL where the product manager introduced complex new features to justify their salary rather than for the benefit of any user.
Haven't played around too much yet, so I'll comment on new features when I stumble upon them. So far, I like the drag-and-drop in admin/build/block (thanks Nate!)
