Dave NX2 is a more powerful version of ViewNX. Think of NX2 as Nikons version of Photoshop.
As I just switch to a digital camera my experiences are limited. The reviews suggest that except for batch processing NX2 is a very good program--it is also significantly less expensive than photoshop about $120 to $170 vs $500.
If you want to learn how to do more sophicated editing ( such as editing small parts of a picture etc) NX2 is the way to go. If you just want some decent quickie fixes stay with ViewNX
For Nikon users NX2 and ViewNX has the advantage of being able to edit the RAW file(which is typically 10Megs or more per picture vs 3Meg max for a Jpeg file).
When edits are made a copy of the original is preserved.You can work on the RAW File ,Jpeg or Tiff file.
If you have a faster computer I would suggest editing the RAW file as it contains all of the information (every pixel etc) picked up by the cameras sensor.Jpeg is smaller but you lose some of the fine definition that might prove useful when editing.
The NX2 CD comes with a manual. Also the program has a help tutorial .Once you have read them I might suggest you obtain some of the NX2 books.
The CD contains NX2.0. After you load it go to the Nikon site and load the latest version.There is also a 60 day trial version but you need to pay full price to continue using it after the trial period.
The one thing I like about ViewNX is that you can scroll thru full size pictures and then decide which ones to eliminate.After I have done this I go to NX2 to do my editing.