I have several sites running on Drupal (which is great). Just after D7 came out, I started using it as the basis for all new sites I set up. But recently, I upgraded 2 of my old D6 sites to D7 and both upgrades where a bumpy ride.
The first upgrade was on a site that started on D5, was upgraded to D6 (rather smoothly). When I ran the upgrade to D7 several times, the site crashed horribly with even core modules like color throwing errors. This wan not a big site, so I set up a clean D7 install and copied the node content over.
Yesterday, I did my second upgrade, this time from a pure D6 database. It looked ok, until I noticed that each node was missing it's content. I ran the upgrade 3 times to be sure, I checked the database and found the content there, but it was not showing on the site or when editing the nodes.
After a long search and trying several things, I found out that my problem was directly linked to the content translation features that were active on the side, and I had to run some manual SQL queries to get 'und' type translations back for the nodes. The full explanation is here.
It will be an interesting ride to upgrade the remaining sites, but I'll hold off a while until it's actually needed...