Today marks the 5th stable release of CiviCRM 4.3. The CiviCRM community has truly rallied to make 4.3 the most reliable and feature-rich version yet - over 60 people contributed patches and testing to 4.3.4 alone.
This is a security release. You should upgrade your site immediately. If you are unable to do so, read the following security bulletins for alternate instructions for securing your site: