This is a security update, it is important to apply it as soon as possible. It resolves multiple vulnerabilities and has a few minor bug fixes as well. I know some of the bugs being fixed here have affected clients so this is valuable both for security and the fixes put in place.
WordPress Update Process
Anyone that is set to receive minor or major/minor core updates on our system will automatically receive this over the next 24 hours. Our systems are starting to update sites now.
Updates from the WordPress auto-updater are also rolling out as I write this.
Minor updates like this are less likely to cause problems, and given the nature of this update, I would encourage you to update immediately if you don’t have auto-updates turned on.
If you haven’t updated to WordPress 4.4 yet, this might be a good time to do that as well, as this update is taking care of some of the issues 4.4 caused, and most plugins/themes have been updated for quite a while at this point. The security update will be ported back to the last few major version branches as well so you don’t have to hit 4.4 to have this (4.3.2, for instance, was just released as well).