SQL Server 2008 R2

The R2 upgrade is mostly focused on data warehousing and BI features. They are incorporating some technology from a few companies that they purchased. You can read more here: http://www.microsoft.com/sqlserver/2008/en/us/r2.aspx

If you have a need for the items, like more than 64 logical processors, or are heavily using some SQL Server 2008 specific features, it might be worth it. If you are upgrading from 2005/2000, I'd go with this one as well since it extends out the support, and it's a "smoother" release of 2008. They've fixed some things from 2008, and it should be more stable. Think SS2K8 SP3.

However, if you are trying to just keep current, I think you would be better focused on learning more about how you can use SS2K8 in your environment rather than spending time testing an upgrade.

This release has a few interesting incremental features (some nice SSRS enhancements), but mostly it appears to be trying to get a few customers with the data warehousing enhancements, make some large BI customers happy with the > 64 processor enhancement, and set a base platform on which to build for SQL 11. I'm not sure I'd jump onto too many of those new platform features here as they'll be 1.0 releases.

In cases where you've purchased SQL Server with software assurance (SA) , or you have a SPLA license then you don't have to pay for upgrades.