One of the big new features of Leopard, the current release of Apple's OS X is Spaces, and I was really looking forward to using it. Unfortunately, the actual details fell short of my expectations, and I have turned it off. I've used similar features before (under X Windows, for instance) and being able to organize a larger virtual desktop can make handling many simultaneous open windows easier.
I currently have 22 application windows either open on my desktop, or collapsed in the dock. It's not unusual for there to be 10-15 open browser windows, but right now I have only 8. I have a couple of terminal windows for talking to remote computers, three emacs windows (two currently collapsed), Word, ITunes, two for Numbers (Apple's spreadsheet program), one Preview pane, Idea, Thunderbird, and Shrook (blog reader). I'd expect to separate tasks by project, so I might have a Space for tracking Real Estate, one for working on Zocalo, one for reading blogs and web pages, and so on. Without Spaces, each shelved project takes up several slots in the dock, making it harder to find whatever I'm looking to do next.
When I attempted to navigate between applications using the keyboard, Spaces would throw me around somewhat arbitrarily, breaking whatever train of thought I had going. If I used keyboard commands to switch applications, I expect the system to choose a window for that application that is already open in the current space, or if there are none, to allow me to use Command-N or Command-O to open a new one. Instead, it would arbitrarily choose an open window for that application and switch me to whatever space contained it.
In the end, I decided that I'm better off with a cluttered dock, and a single desktop than with their implementation of Spaces. Maybe the next version will do better.