MSDN magazine just posted a great article about NGEN, written by Reid Wilkes.
It is a wonderful read if you want to know how NGEN works, and how to use NGEN.
The article is here:
NGen Revs Up Your Performance with Powerful New Featureshttp://msdn.microsoft.com/msdnmag/issues/05/04/NGen/default.aspx
Reid also started blogging recently. His blog can be reached at http://blogs.msdn.com/reidlw.