I’ve been trying out Drupal 6.0 and it looks like this will be one of the most painful upgrades. I tried it with a local copy of MacMegasite and I found that it broke almost everything.
I don’t use too many modules, but I use some pretty basic stuff like user points, buddy list, bbcode filter, adsense, service links, and twitter. None of them are compatible with Drupal 6 yet. It’s been in development for over a year, and most modules still haven’t been upgraded to work with it.
I avoided using betas of 6.0 because of the module compatibility problems, and I find that the problem still exists in the release version. It looks like I won’t be upgrading any of my sites too soon.