You should be able to upgrade all your 5.3.2-and-greater sites to it without any problems. There have been a lot of enhancements to concrete5 since 5.3.3.1. I've tried to list them all out here:
Additionally, I've updated all the checked-in language files in our Translation SVN Repository with new strings for 5.4.0RC1. To those of you maintaining a translation, you can update your own local copies and work with the new strings. There's about 250 new ones or so.
