I'll often be asked for solid information about the performance of SQL Server 2008. Even though it is not yet completed and there is more performance tuning going on, the SQL team has published the results of February 2007 community technology (CTP) release. The results were published around the time of the first launch events. The results are posted on a SQL Server 2008 CTP performance blog entry.
The results include performance against industry standard benchmarks. Although that's important, there are also some real world tests that are published including transactions per second and decreases in space requirements. More results are promised as the release nears.
You can find out more and can watch the progress of SQL Server 2008 performance by following the SQL Server 2008 Performance blog. You can get started with a SQL Server 2008 trial version too and try out your own benchmarks.