This is true, a very-fast screen switcher mitigates the cost heavily, maybe even negates it entirely. Alt-tab never did it for me, too unpredictable. On OSX, while developing with a single monitor, I pretty often split my things between 4 desktops, and it's great.
On Windows, on the other hand, nothing switches quickly. Ever. And it takes a long time for applications to "wake up" after even short periods of non-use. Nothing but a second monitor has ever saved me.
On the iPad, currently, the situation is far worse than even Windows users suffer through. Double-tap the home button, and click the application (maybe scrolling between), which isn't in a static location. That's a non-screen action, followed by a change in the UI, followed by a touch, followed by a comparatively-slow transition. I liked someone-here's suggestion of gestures solving this, but single-tasking on anything without a separate keyboard is a total efficiency-crap-shoot everywhere, from what I've seen.
The issue that I have with desktop switching on my shiny new MacBook is that OS X switches focus between applications, not windows.
Here I've got vim and Firefox open, with the focus on vim. I switch to another desktop to glance at some documentation (since the Firefox window on the first desktop is for the application I'm developing), switch back and begin typing. But alas! My key strokes are logged to Firefox, because that was the last application I had focused, rather than to the last window I was using on the desktop.
Aside from that, once I added some basic tiling functionality[0], it's not too bad working on it, even for a someone as used to a tiling wm as me.
On Windows, on the other hand, nothing switches quickly. Ever. And it takes a long time for applications to "wake up" after even short periods of non-use. Nothing but a second monitor has ever saved me.
On the iPad, currently, the situation is far worse than even Windows users suffer through. Double-tap the home button, and click the application (maybe scrolling between), which isn't in a static location. That's a non-screen action, followed by a change in the UI, followed by a touch, followed by a comparatively-slow transition. I liked someone-here's suggestion of gestures solving this, but single-tasking on anything without a separate keyboard is a total efficiency-crap-shoot everywhere, from what I've seen.