I am just amazed by the amount of people agreeing that Leopard is si much faster than Tiger. Yes it takes a bit more memory to run but honestly, it is much unlike the Vista upgrade where you need to buy the best of the best on the market. Even some friends with powerbooks and ibooks agree that Leopard is a bliss, and those machines are at least 2 years old!

Some more info:
[The Gadget Blog - Leopard is fast]
[Ars Technica - The Kernel]