I had a chance to try Lion at a friends house. He has two screens connected to his MacPro. The really annoying thing is that if you put an app into full screen mode the second screen is useless. Whatever app is displayed on the second screen is covered by a gray background image so that you have no access to that app (as long as there is an other app running in full screen mode). Also if an app window is on the second screen and you put it in full screen mode it jumps to the main screen (the screen with the menu bar) and renders the second screen unusable. So for people with two or more screens connected to their Mac this feature is basically useless.
To illustrate this, here is an example: In Snow Leopard my friend often used to play some music video in full screen mode on his second screen using QuickTime X while at the same time doing work on the main screen. In Lion, QuickTime X refuses to go into full screen mode on the second monitor, it only goes to full screen mode on the main screen, and of course the second monitor is inaccessible.
So it seems Apple made this feature useful only for single screen setups.