Average fuel economy is the same as 2007 Pontiac G6 Sedan, despite a lower weight and weaker acceleration. ~28 MPG with more highway than city driving.
I drove to Death Valley on a cool high-pressure day, and the engine revved very lively, almost matching the acceleration of my old '94 Protege LX (which averaged 30 MPG BTW). No cigar on the hot Nevada highlands though - it definitely had breathing difficulties and petered-out around 5600 RPM. The redline is 6500.
The drive-by-wire throttle is buffered for smooth take-offs. It had a metallic chirp when accelerating onto the freeway at high RPM. Could be an exhaust shield, although more like a maladjusted timing chain guide. A private owner could probably get it fixed under warranty. Too bad for the rental company though.
AFAIK this engine is a descendant of the old 2L OHC Isuzu/Pontiac that was used in Sunbirds and that little korean hatch. By comparison the new 1 has weaker mid-range pep. Maybe it had a dirty air filter, but I could not check it without a set of tools (hint). The headlights require Torx bits to adjust.
GM had a real bad Torx fetish in the early '90s - Cobalt still has some of that. On my '91 Regal the headlight screws were so stuck that I broke 2 Torx bits while attempting perfection. I ran out of bits and the light was not too far off anyway.
The ride is firm, but not harsh like the G6. Throw down a thin seat pad and you can drive it long-distance. Still a far cry from a classic American though.
My friend had an '89 POS Cavalier that I fixed a few times, and it had a much softer ride.
The handling is a redeeming quality of the Cobalt, almost as tight as my old '94 Protege LX. Mine had the optional alloy wheels with flat-profile tires. when Cobalt loses traction it happens smoothly, with slight understeer and a lot of squealing to warn the driver.
Anti-lock brakes are excellent and allow good steering control when sliding through a curve, although after a few miles of mountain twists they get crackling hot.
The interior has random plastic rattles. Dome light is real cheap and is hard to switch. I turned the headrests around so they don't constantly poke the head forward. They match the seats perfectly this way too. Same on Pontiac G6. Looks like nowadays they are designed to be turned around by the customer. To unlock, push a thick paper clip wire into a small hole on the side of the plastic retainer. Cobalt seats are simpler and more comfortable than G6's, Regal's, base Plymouth Sundance, or Cavalier. In fact they seem to have been copied from the '90-94 Japanese imports.
The interior layout is better than G6 though. For example, the cupholders are at the bottom of the center stack. The steering wheel is placed farther forward than usual, but it is the size of a computer gaming wheel and does not get in the way when getting in & out. Thumb buttons are useful.
The radio had an occasional glitch that I would fix with a firm tap :-) Seems to me they are using the good radios in the premium models like the G6 and the 2nd-tier rejects in the cheap models.
Power windows have weak tracks, so that when fully raised the glass pushes outward and opens a void to the inner weatherstrip, through which cold air blows in and chills the shoulder uncomfortably. My mother's 1992 and 1994 Suburbans had identical problem. Continuing the bad 1960s tradition, this car is built for profit rather than excellence and reputation.
IMO this Cavalier is not as good as the early 90s Pontiac Sunbird. I am a mechanic as well as an engineering hobbyist. In my experience GM got desperate for profits in the late '90s and are cutting corners everywhere.
Overall I would not recommend this car for what it costs. You are better off buying a used '90s Japanese import or a late-80s Benz.