... for cross-platform development. Is it worth it and can it even be done for current high-level VB6 applications with sophisticated 3rd-party controls?

A sponsor is pushing hard for both Windows and Mac support. I'm getting mixed-bag advice on this. Most say it is virtually impossible today to build complex applications that do the same thing with the same look and feel on both machines. Does Java even have similar VB6 controls?

In somewhat serious mode, what say you?

Note to Mods: Please move thread if appropriate. I may have misjudged the salience.