There have been a few significant changes made since the second Release Candidate, including the removal of AJAX list tables, which introduced too many bugs and usability issues to be considered stable for 3.1
Because of the major changes, the developers are asking for as many beta testers as they can get.
If you run into any problems, please feel free to discuss them in the Alpha/Beta section of the Support Forums or contact the wp-testers mailing list. If you run into any bugs, please follow this guide to report them.
Have you tried WordPress 3.1 RC3 yet? If you have, what do you think so far? Are you ready for the final release of WordPress 3.1?