Will NTFS-3G and FUSE work out of the box if I upgrade from Snow Leopard to Mountain Lion?
Following that major upgrade of the operating system, you may find that some features of your product combination (NTFS-3G for Mac OS X, with a 2.2.1 version of the MacFUSE framework) are: 


usable, but unreliable, with a risk of kernel panics. 


Recommendation

Install FUSE for OS X 2.4.2 or greater, with its MacFUSE compatibility layer: 


before or soon after the upgrade of the operating system
certainly before you attempt to use NTFS-3G or any other software title that requires MacFUSE. 


Background

Maintenance of MacFUSE, in the Google Code area, ceased in 2009. 

Successors to MacFUSE include FUSE for OS X, also known as OSXFUSE. The first version with experimental support for OS X 10.8 was 2.4.0 (2012-05). 

In my experience with OSXFUSE 2.3.8 and greater on multiple builds of Mountain Lion:  


OSXFUSE 2.4.2 should be good for the vast majority of use cases with released Build 12A269 of the operating system
a delay-related alert may appear whenever an NTFS volume is mounted with NTFS-3G (2010.10.2) – this alert, not specific to Mountain Lion, may be dismissed.