With Plone 4 being released for quite a while and with some free time at my hands I took some initial stabs at a migration or update of my site which is currently running Plone 3.2.x. Here, some random notes about the first results:

• The current theme won’t work. Peroid. This theme was created using the standard approach with paster. And I still don’t have any clue what need’s to be changed to remedy this. As an alternative, some xdv-based theme might be an option.
• If your migration script fails and shows you some error regarding “Object not found”, then a broken index might be the issue at hand. And if you still happen to have one of the old TextIndex.TextIndex” indexes in your catalog, a simple rebuilt won’t help. Better delete the corresponding index and replace it with a ZCTextIndex index, rebuild, voila, see my post Upgrade from 3.2.2 to 4.0.2 fail on the Plone Setup mailing list.
• Currently, all pages here are written as restructured text. With the advent of TinyMCE as editor, I might even go for a graphical editor. For this purpose, yet, all existing documents need to be converted to the html-format. More information? See here (in german)
• After migration, the workflow state needs to be fixed somehow. Currently, all items show up in the review list yet even if they do have a “published” state. Retracting and republishing is not an option as then all the creation and modification time data etc. will be changed. This is still an item for further evaluation.

Anyway, this concludes my personal experiences. So much for now.