A vivid analogy about whole fruit vs. juice:

Think of your all-time favorite rock song: Hey Jude, London Calling, Smells Like Teen Spirit, whatever. Now imagine that the next time you crank it up, all the guitar riffs will be replaced by violins. Kinda weak, right?
 
Well that’s akin to what happens when you turn a fruit into fruit juice: You still have the flavor, but you don’t have the grit, the substance, the power. Even the very best fruit juice isn’t as nutritious as the fruit it originally came from, because the fiber that makes a piece of fruit so filling has been stripped away: Instead of filling your belly like an apple or an orange, juice just passes through your gastrointestinal tract like a little stream of sugar. It’s like listening to “Hey Jude” without the “Na-na-na na” part at the end. The sweet melody is intact, but the soul is lost. So as a rule, always choose the original version (that would be the fruit) over the Muzak version (that would be the juice).

(from Men's Health "Unhealthiest Juices in America", which goes on to undercut that premise by presenting some juices, lemonades, and drinks as reasonable. As part of their "Eat this, not that!" series, they highlight six terrible pseudo-juice products and suggest alternatives, most of which benefit from portion control as well as more healthful ingredients.)

(Loosely based on their picture)