I love OSX Leopard, especially the spaces feature. It is really nice to have several "Spaces" with different apps open on each. However, I did find one nuance that is a bit annoying. If you open an application which allows for multiple windows (such as Firefox), you must use all of the windows within the same space. That means you cannot open Firefox in Space 1 and then go into Space 2 and open another FireFox window.
Now, there is an easy work around to this, and that is simply opening up all of your Firefox windows within the same Space and then hitting F8 to show all Spaces. Once you have all spaces showing then you can drag the windows into separate spaces as you wish. The issue I have found with this is that if you use the dock to navigate and hit the Firefox icon then spaces may go berserk. I have had spaces go back and forth between windows until I intervened with a mouse click.
This becomes frustrating if you are trying to use an technology such as Java Web Start in one Space and another Firefox window in a separate space. If the Web Start application tries to fire another window (such as a report), then Spaces will go berserk again.
Really, this is not a big issue but I thought I would point it out for anyone who was having issues with Spaces. The easiest work around in this case is to only use Firefox or other such apps within once Space only.