I installed it today and all my mxj externals are working fine. But I'm not doing anything too crazy with Java, so you never know. If there is a bug it might only affect some very specific APIs. Use the usual caution when upgrading.
And in case anyone was getting their hopes up, this "release 6" is just an update to Java 1.4 and 5. Still no word on when Apple will release Java 6 for OS X.