They cite clarity as a major advantage of F#:
"We liked that Visual F# is tailored to highly mathematical problems, helping programmers work more closely to the problem domain and enabling actuaries and other nonprogrammers to review the code as they would a math formula,"
"We liked the .NET Parallel Extensions available through F# for managing parallelism automatically through the runtime system."
and integrated development environment and interoperability with existing technologies:
"We also liked that Visual F# is an integrated part of the Visual Studio tool set and the .NET Framework. That meant our programmers could use the same IDE and powerful .NET libraries they knew from having developed the earlier version of the solution in Visual C#."
For other F# success stories, read the freely available first article from The F#.NET Journal.