Taken from the an article on TheServerSide.com entitled Microsoft Responds to Sun’s Web Service Benchmarks

In a paper published last month, Sun claimed that Java based web services outperform .NET based web services both in throughput and response times. Microsoft has released a paper on TheServerSide.NET responding to those claims stating that Sun’s representation of the .NET performance was understated by 2 to 3 times and that in many, but not all cases, .NET exceeded the Java benchmarks.
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Read the Microsoft response on TheServerSide.NET: Web Services Performance: Comparing J2EE and .NET

Read Sun's original paper: J2EE claimed to have better Web Services performance than .NET

It should be noted that Sun did not publish source code for their benchmark so Microsoft had to  re-create Sun's benchmark based on the details from the original paper. The Microsoft response has the full source code for both the .NET Web Service implementation, the Java Web Service implementation using Sun's JWSDP 1.4 along with the test program used to benchmark both services. I always believe the best way to verify a benchmark is to run it yourself. The performance of the .NET XML Web Service implementations should prove to be a lot better than what is implied by the original paper from Sun.