Understanding what Silverlight / Moonlight brings to the table is a good starting point for making a rational assessment as to its value. From the Moonlight website the stated goals of the project are:
Silverlight 1.0 has a number of basic capabilities focused around multimedia and Rich Internet Applications (RIAs). Scott Guthrie has a good blog post that describes what Silverlight 1.0 entails. Version 1.0 is somewhat limited since it does not include support for the .Net framework.' To get that you'll need Silverlight 1.1. Moonlight 0.8 does provide some support for Silverlight 1.1, but it's still a work in progress.
With Silverlight 1.1 Microsoft delivered support for Visual Basic .Net and C#. It also provides support for using the Windows Presentation Foundation (WPF) programming model. WPF uses XAML (eXtensible Application Markup Language) to describe the user interface elements and a .Net language to implement the logic to respond to user actions.
Microsoft is currently in the final stages of the beta cycle for Silverlight 2.0. At the same time Novell is finishing up their beta of Mono 2.0. Once Microsoft releases Silverlight 2.0 the Novell folks will have a firm target to shoot at.