I still haven't upgraded to Leopard. Call this prudence. So far the reports I have read have been mostly positive. Many were disappointed to see that Leopard didn't ship with Java 6, but it looks at least like Java 5 is running pretty well on Leopard.
With one exception: in some cases (all cases?), you can't switch between multiple files you have open in IntelliJ. This unfortunately is a major issue. One workaround is not to use the Mac OS X look & feel. I am not sure if this is a complete blocker for an upgrade to Leopard, but I will sure wait a little longer and see what happens with this one.
Update Nov 13, 2007: Great news: Dmitry mentioned in a comment that this issue should now be fixed in the 7.0.2 EAP build. Thank you Dmitry. For sure, those IntelliJ guys aren't sleeping!