The 10.9.1 update, a 243MB download, should be welcomed as an early Christmas present for anyone who uses the build in Mail application with Gmail. It was widely reported after the initial release of Mavericks that there were issues keeping email in sync between the client and the Google servers. This update should address the Gmail issues. (I don't use the Apple Mail application, so I can't really comment on the issues Gmail users were reporting.)
In addition to the Mail update, Mavericks 10.9.1 also updates Safari to version 7.0.1. The latest version of Safari fixes an issue where Safari could become unresponsive when filling out web forms and improves the credit card autofill feature's compatibility with websites. (I have been using the credit card autofill features this Christmas season and it's been working well.)
This latest Mavericks update will install on any Macintosh that is capable of running OS X 10.9, including some models going all the way back to 2007.
A full list of changes, fixes, and enhancements can be found on the Apple website.
If you haven't already done, so, you can download and install the Mac OS X 10.9.1 update on your Macintosh by going to the App Store icon and clicking on the Updates tab.