The S5 is less than three quarters of an inch at its thickest and weighs 2.6 pounds--impressively light for a notebook with a 13.3-inch display. In almost every way, the S5 fulfills the promise of the Ultrabook as articulated by Intel: It's extremely portable, very fast, and endowed with decent battery life.
True, you can find several Ultrabooks with better battery life, and maybe one or two with superior performance--and the Aspire S5 has its fair share of minor drawbacks. But none of the ones we've seen are thinner.
That's thanks in no small part to an innovative motorized panel that Acer calls the MagicFlip, which rolls down to conceal ports on the rear bottom edge. This both protects them when not in use and slims down the S5's profile so it's both thinner and lighter than the current 13.3-inch MacBook Air.
But the motor makes a somewhat grating noise, and sometimes it seemed to roll up of its own volition. Also, I worry that the motor, activated by a button on the top right of the platen, adds one more part that could break.