Last night, I figured (completely irrationally) that I would take the jump and upgrade to Leopard. Here is my first feedback after one day of work with it:

• I got the infamous "blue screen" upon restart during install. I followed option C and luckily thing went smoothly after that. I don't know yet which app installed ApplicationEnhancer.bundle.
• As I already knew, Java windows don't play nice with Spaces. All the Java apps remain stubbornly in the first space and won't move. Also, if you are in a space other than the first one and cmd-tab to a Java app, nothing happens: you have to explicitly go back to the space containing the Java app. It's not a showstopper, but it should be fixed.
• IntelliJ has some random problems with the tabs in the files section: they sometimes, behave erratically. This is annoying and I hope that IntelliJ >= 7.0.2 will fix this. It seem that changing look and feel may work around the issue, but IntelliJ then looks utterly ugly.
  
• Parallels seems to run fine, except that Windows drives don't show up in the Finder.
• I had no immediate obvious issues with Firefox, Thunderbird, or Tomcat.

So I can't really say "so far so good", but it seems like things should be workable.