Having used it on Piera's eMac, I can say that I really don't find it useful. In the world of Virtual Desktops, Expose doesn't do much for me. I mean, seriously, if you have a disoragnised desk, than it probably makes sense, with little notes all over the place. This reminds me so much of "Where's Waldo". You know the book, the one where you have a thousand illustrations of people in an area, and you have one guy, Waldo, who you need to find.
Expose feels like that. Sure, when you only have a few windows open, it is probably nice. But ALT+TAB is so much faster. I can go back and forth without a problem. Expose might allow me to take a look at everything I have open, but I am only looking for one thing. And Expose doesn't know what I am looking for. With Virtual Desktops, I find it much easier to say "Hey, Email goes here on this desktop, generic web browsing here, development stuff here, graphic stuff here,..." and so on. I keep my life organized.
No doubt that Expose is a neat feature, it looks cool, and people probably use it. But as I see it, it's a Band-Aid on a larger problem. Window organization. Taskbars are the same thing. The computer can multitask, but the user is only ever actively using one application at a time.
No, really, that's it. Even if you are using a PHP IDE and checking the results in the browser, their is only ever one active application. You are writing in your IDE, and then switch to the browser. This makes a Virtual Desktop environment much more suited to getting stuff done. A virtual desktop allows you to setup "workshops", and in this workshop, you can get all your tools out ready to use. You are only ever going to use one tool at a time.
But with something like Expose (and as far as I know, Mac OSX doesn't have virtual desktops), it's like having one workshop, with all your tools laid out at once. You are only ever going to use one at a time, but now you have all your projects in one workshop. This means your life is much less organized, and you have just increased the number of choices you have to make whenever you want to switch tools.
On a side note, Piera will probably beat me up for all of this. After all, she loves her Mac like any good Mac Addict does. And she freely admits her addiction.