Leopard works fine for me... for others, not so much. It's impossible to say that Leopard is faulty, since by the scientific method, one example of the contrary disproved the entire theory -- and the fact that many, many, many people are running Leopard flawlessly disproves the theory that Leopard is inherently faulty.
We could try and troubleshoot the problem for you -- your Mac Pro should not be crashing as much as you say it is. It could be any number of problems, but I'm positive there are people in the world running the exact same model of Mac Pro and Leopard without issue.
Apple offers plenty of outlets to get help with your problems. You could bring your Mac Pro to any Apple Store and speak with an Apple Genius about the crashing problem -- if you can demonstrate your problems in front of the Genius, then they will be helpful and courteous and assist you in rectifying the problem.
Without going through the proper channels first, though (you, as a consumer, have a responsibility to make every effort possible to resolve your problems first, which includes having a certified Apple specialist look at the computer under warranty and giving Apple ample time to either fix or replace your computer), then your class action lawsuit is moot. I suggest cooling down, taking a breath, and being patient -- we can help you to try and get your computer into a stable state as it should be, and, barring that, you have other channels (warranty repair, Apple Geniuses, etc.) to go through first before you can continue with a lawsuit.
However, if you do feel that Mac OS X and your Mac Pro are just plain crap and that Windows provides a better and more stable overall experience, no one is chaining you to the Mac -- simply return the computer for a refund and purchase a nice, new Windows machine.
You could also load Windows directly onto the Mac Pro, too.