So what are the whizbang magic features?
In addition to the already mentioned adjustability, the Weber is supposed to do a proper job of mixing fuel and air at a lower pressure differential across the venturi than most carbs, and was considered to be the next best thing to fuel injection, which requires no pressure differential at all. The emulsified fuel from the main circuit is delivered through the "bomb site" in the auxiliary choke, in the center of the airflow where the velocity is high, rather than the needle jet at the bottom of a slide carb, where the velocity is low.
I have accurately measured 51+ MPG over a 700+ mile trip, and have noted mileages as high as 60+ at lower speeds.