While only a half pound heavier, it makes a difference. Its smaller footprint is nice for space but when you consider it is heavier, with 4 less hours of battery life, is thicker and less comfortable on the wrist for typing, and runs much hotter than the MBA, it is hardly the same.
The rMBP is great for a pro laptop and the screen is beautiful, but it does not match the Air in design or comfort of use imo.