You've probably heard: OS X 10.5 (Leopard) runs Java exceptionally
slowly if at all. This is because they've turned off OpenGL hardware
acceleration (or what little of it they had in 10.4). I've been told
that a number of Java simulation environments, such as Processing,
are having difficulty. I have not tested MASON under Leopard and
cannot verify that it will run well or not. But given Leopard's
serious Java problems, you may wish delaying any upgrade until a
later version (10.5.1, perhaps).