Well notebooks are very good for the average person (and by that, I mean adults, not kids).
They're portable, offer good power and battery life, and if engineered right, don't weigh as much as they used to.
However, they are usually a lot more expensive for the same processing power, are less upgradeable, and have a built-in expiration date with their displays.
However, if you are a pro and run some intensive stuff, a desktop is a must. Email/web/iLife/iWork isn't very heavy and can be done on notebooks that are more than 3 years old.
You can't expect the same kind of return if you're running a business that uses multi media apps. For that, a desktop is clearly the better option.