They clearly want to make it like the iPad and this is how the ipad does it.
Then in my opinion they're focusing too hard on bringing elements from iOS in and not focusing enough on how different the platforms still are, and where to draw the line between making it "iPad-like" and keeping full functionality as a computer OS.
Using Spaces on SL, set to the bottom-left hot corner, switching between spaces can be done with the flick of my wrist and and a click. There are no new spaces appearing because an application is full screen, so I know where all my windows are. It's all done in a minimum of time and as Apple are so keen to tell us, it simply works. It can be done without leaving either the keyboard or the mouse/trackpad, and barely disrupts any working flow.
On Lion, to access spaces, I gesture to bring up Mission Control, find and click a new space, then click a window in that space to exit MC. I've found it far slower, and so it gets in the way of what I want to be doing. Having an app full screen just makes it worse; I frankly find a new space appearing to be pretty confusing.
Personally I never found any problem with opening spaces using the hot corner, then four finger swiping down to Exposé all apps. Essentially the same as MC, but I only need to go to the extra effort when I need to.