A full cumulative update for .NET 3.5 in a form of a service pack. But be carefull – it may break all your bases belongs to us like .NET Framework 2.0 Service Pack 2, .NET Framework 3.0 Service Pack 2 or .NET Framework 3.0 Service Pack 1. The problem resides in the fact, that it no only contains many new features building incrementally upon .NET Framework 2.0, 3.0, 3.5, but also includes cumulative servicing updates to the .NET Framework 2.0 and .NET Framework 3.0 subcomponents, which lead to these problems. So this is -1 for installing it. But… not only performance improvements are inside, but service release flavor ones as well, like opting out of strong-name verification for fully trusted assemblies, ClickOnce opt out of signing and hashing, filestream and date support for SQL2k8 in LINQ to SQL, etc., this would be a ++1. So – test your applications throughfully, it worths upgrading, but may hide many obstacles…