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"Right, but apart from that: what does it actually do that is useful?"

Provides more screen estate; better navigation between applications; provides accessibility features like creating a more high contrast desktop; Zoom into an object those applications. The power of tabs added to every applications. Better desktop management in a multi-user environment. Attractive working environment. Small applications that report anything from system information to the weather. Better windows management that have only been available within an application ; tile etc. Preview of large application useful for checking whether batch job has finished. Real positive visual feedback to basic windows tasks, for minimize; active application; alerts.

...and this is of the top of my head.

This is ignoring all the potential applications like a *real* desktop analogy; none standard windows. Already applications are coming out that show how photos can be views sorted.

...but the bottom line is why the hell would you not want you windows to fold into a paper airplane and float away; simply because its attractive; fun; and a little exciting which is what computing should be about. If nothing else its looks modern.

"why the hell would you not want you windows to fold into a paper airplane and float away"

Because it's slower than if they just disappeared instantly ?
And, you know, some of us use the computer to get the job done, not just to impress friends

It looks neat, but so far I've found it objectively doesn't really offer any increased productivity.

"It looks neat, but so far I've found it objectively doesn't really offer any increased productivity."

I'm not disagreeing with you, although objectively you have little to know way of knowing. Even proper testing I suspect would only produce dubious results.

I suspect a faster CPU, more memory, or even better a larger monitor...or two will have a greater effect on productivity, esp as in this instance we are talking small amounts of time that add up.

What is clear is that it does make desktop *events* clearer; improve your screen real estate; provide smooth transitions between events; add accessibility features.

Pretty desktop background don't offer increased productivity either, but most people seem quite fond of them despite this. People will also argue about if a certain icon set is pretty or ugly for hours at an end, yet no one has shown objectively that a pretty icon set increases productivity.

And to round it off, while I have no idea if expose on OSX has actually increased my productivity or not, I find it a great feature which makes the computer more enjoyable and easy to use.

Alt-tabbing between windows is suddenly more useful when you can see what you're alt-tabbing to (Ring Switcher). Expo lets you easily juggle windows around your virtual desktops. And the window tabbing plugin makes it easy to group 20 windows you only use every now and then to 5 windows.