But J# code neither runs in a Java VM (virtual machine) nor leverages run-time features created after Version 1.1.4 of Sun's Java SDK (software development kit). Microsoft is banking that the differences between Java and J# will be obvious to lawyers and judges -- and that J# will give Java serious competition for the minds of Java developers.
--http://www.infoworld.com/articles/tc/xml/01/11/05/011105tcjsharp.html
so it's not Java, so why use it? Safer to use C#, which is Java-like enough and less likely to be shut down in another lawsuit.