Unlike Microsoft's FXCop, which was largely celebrated as an important step towards improving the consistency and quality of .NET code, StyleCop has been viewed with much suspicion. The main difference between the two is that FXCop focuses on compiled code from any .NET language while StyleCop works solely against C# source code.
The biggest complain about StyleCop is that its recommendations are mainly based in opinion. While some of the guidelines in FXCop are subjective, most are grounded in sound logic based on a deep knowledge of how the CLR works. StyleCop, on the other hand, is mostly about hotly debated issues like how many spaces to use for indention. Some are even downright contrary to standard practices like placing "using" statements inside namespaces.
With the release of the StyleCop SDK, developers can develop their own rules to supplement or outright replace the default ones. While in the long run developers are going to want to be able to simple configure the rules to match their company standards, this is at least a good temporary solution.
In addition to creating new rules, developers will find information in the SDK on how to integrate StyleCop in MSBuild tasks.