Despite worrying, planning, and putting-off upgrading this site from Drupal 5 to 6, the actual upgrade took less than an hour, including the backup and migrate. There was only one puzzling hangup. After 15 minutes of searching, I realized that I needed to turn on the taxonomy mod and everything finished upgrading without errors!
I'm still upgrading modules and turning them on, so some site functionality will be missing for a while. I will also take some time to re-architect some of the site and maybe finish some ideas that weren't easy in D5.
My custom theme isn't D6 compatible, but I really wanted to redo it anyway. For now, I'll be a plain-jane Zen site.
Overall, I'm very pleased with this upgrade path. It gives me more confidence in using Drupal for long-term projects.
It's late now, but before I leave I want to thank all those in the Drupal community who made my life so much easier today! Thanks for automating so much of the complicated pieces! And thanks for making D6 (& D7 even more) something so great that I just had to risk upgrading!