I migrated (lie the majority if Hudson users) to Jenkins. The migration is easy and simple (backup, add jenkins deb in sources.list, install, the rest if fixed by Jenkins). All info for migrating users here. While Jenkins regained its frequent release cycle, former Hudson is still sitting on version 1.398.
I want to highlight the first (and only) book about Jenkins CI by John Smart (link): You can download it for free. Thanks !