I have lots of friends who use Java, including myself sometime. So, out of curiosity, I googled "Asp.net Vs Java" and stumbled over this interesting discussion. I really can't put my few cents over how J2EE will perform against .Net as my expertise is mostly related to .Net, but reading the threads gave me some insight. Check it out and you'll enjoy it too.
http://philip.greenspun.com/bboard/q-and-a-fetch-msg?msg_id=000tcP
Here's another test done by Mujtaba Khambatti on JSP vs ASP.Net.
http://www.devhood.com/Tools/tool_details.aspx?tool_id=793
I would definitely like to hear some feedbacks from both Java and .Net developers. My intent is NOT to pick up a fight and render one vendor superior than other, but actually get a better understanding of which platform or programming language might be better in different scenarios. I am a big proponent of open source and enjoy both the platforms, so would be interesting to see what others think as well.