Both Express and Pro have had a few bugs fixed, some enhancements, preliminary support for Mavericks (OS X 10.9) and other niceties that are too numerous to list here. You can read the complete release notes for Express and Pro online or under the Help menu in both apps. I hope your chair is comfy and you have some snacks ready because those notes make for a long read.
You can update from within the apps or from the Express or Pro pages. These apps can be purchased from our store. Nisus Writer Express is $45 and Nisus Writer Pro is $79. There is upgrade and academic pricing, and three license family packs are available.
Nisus Writer Express and Nisus Writer Pro require at least OS X 10.6.6.
These updates are recommended, especially if you plan on using OS X 10.9. So please update and enjoy!