I was going to give my opinion the other day when you asked about using on a desktop. "I" feel it would be worth it to green up a desktop. Though laptops would show the most obvious short term gain, a desktop would gain as well. No sense at running full tilt for surfing the web, reading emails, or other low impact tasks. At today's gas prices, you don't keep you gas pedal floored while going to the grocery store. You just go at a reasonable pace. But on the other hand, it's nice to know if you ever see "Blue lights" in your mirror, and happen to have the back seat loaded to the gills with moonshine, you can get the heck outta Dodge if you have to.
Same thing with the desktop. With "ondemand" the processor will run slower and cooler when doing light duty tasks, and then kick it in to high gear if you start doing something more demanding.
I've seen many debates amongst PC "experts" about what is more expensive. Running a desktop 24/7 or booting when you need it. Some feel it's no more expensive to run 24/7 at a constant load compared to booting as needed and drawing larger amounts of power to get it started. So I deducted that the cost of running 24/7 is trivial, and my desktop would be up and ready the moment I needed it. Recently my wife put her foot down, and forced me to shut the desktop down when I go to bed. I had no argument as I don't run servers or anything. We immediately noticed our electric bill dropped just over a $100.00 a month and has been down that much consistently. I still have it running nearly every day during the day. Also, the monitor is LCD rather then CRT.
So if you are able to save real money in any amount, and not sacrifice performance by simply enabling a feature of your hardware. I think it is worth it.
Another point worth mentioning, many users aren't aware that many desktops are capable of sleep/suspend and hibernating. And resuming from sleep/suspend to a working environment can be in as little as three seconds. With open documents, browser windows or whatever, open right where you might of left them.