Nice MSDN article which gives you a good overview on IIS compression:
Designing and Developing High-Performance SharePoint Server 2007 Publishing Sites (works for 2010 as well)
Blog article explaining you how to configure IIS compression
Using HTTP compression with SharePoint
Very interesting read with numbers from the fields.
IIS 7 Compression. Good? Bad? How much?
Properties and configuration possibilities
E – H (IIS 6.0 Metabase Property Mapping to IIS 7)
HttpCompressionSection Class [IIS 7]