Constructor, in many ways, is like Marimba Inc.'s Bongo. They are both tied to a specific class library. So if you build your programs with Constructor and IFC now, you'll be in a better position to leverage JFC when it becomes available.
Constructor gets an okay.
Performance Java programs built with Constructor run well, but don't expect them to run any faster than other well-written Java programs. It is, after all, just a UI builder. Extensibility Constructor isn't overly extensible. You can, however, add your own components to it, making it easy to drag and drop custom components onto your work pad. In general, they don't take their looks from the platform they run on, but instead take on a new, IFC look.