Reasons I think, to Upgrade to Visual Studio 2008

1.Build Applications which can run on and support Multiple versions of the .NET Framework
2.LINQ support to work with Data in a Unified and Integrated way
3.Support to Build Office 2003/2007 based Applications.
4.Amazing Unit Testing support on right click in the code view and dedicated Test Project Template.
5.Ground level support for Windows Vista Applications
6.SilverLight can be done here, which seems to be the latest WEB User Interface Technique
7.Facinating enough to have dedicated Project Templates for WPF, WCF and Workflow

How to Un-Install the previous Orcas CTPs, Betas before installing VS 2008 RTM

You may face some issues while installing VS2008 over the previously installed Orcas CTPs or Beta editions.

You need to un-install all the previous versions before you start work with the VS 2008 RTM.

Let's have look at the steps availabe on the MSDN

Visual Studio 2008 RTM is Available now

VS 2008 RTM is availabe to download for MSDN Subscribers
Get VS 2008