I'm getting great gas mileage, and despite mixed rated for 29, I'm getting 34+ mpg. The gearing is just about right, giving its power (and efficiency) in all the right places -- with the manual trans you won't feel like you have such a small engine (reviews on internet says the auto can feel sluggish). The car has a lot of nice touches -- arm rest, aux input, stub holder, center console orientation, the list goes on. All of the small things add up to get a comfortable ride. I can't think of any downsides, although friends have reported the back seats feel a bit cramped.
I bought it used, and I think when it comes time to buy a new car, I'll go with a Rio (or Hyundai's version, the Accent).
Fuel Economy, quality of build vs. price, overall engineering.
Accelerator -- It kinda has an "on/off" feel, instead of a smooth transition from off to on.
Add a sixth gear.
Improve shifter linkage.
Take a couple inches off of trunk to give to rear passengers.
Give a bit more room between the wheel and paddles for lights/blinkers/wipers. I sometimes hit these by accident.