Oracle/Sun says its fine as long as you don't run it locally on production box? http://download.oracle.com/javase/1.5.0/docs/guide/management/jconsole.html
Note: Using jconsole to monitor a local application is useful for development and prototyping, but is not recommended for production environments, because jconsole itself consumes significant system resources. Remote monitoring is recommended to isolate the jconsole application from the platform being monitored.
Have you ever enabled it in a production environment and experienced any performance impacts?