Interested in upgrading your existing SQL Server 2000 or 2005 instances to SQL Server 2008 R2? If you need advice or assistance then you should definitely check out the SQL Server 2008 R2 Upgrade Guide written by Solid Quality Mentors (SolidQ). The document, which is a supplement to the SQL Server 2008 R2 books online, provides essential phases and steps to ensure a smooth and successful transition. SolidQ are writers for the SQL Server 2005 and 2008 Upgrade Guides as well so you are definitely in capable hands.
If you have not considered upgrading or you are still looking for reasons to upgrade then check out the Why Upgrade Whitepaper. The guide and the whitepaper are available here: