UPDATE: So I'm 18 months in to owning it, and have put 24k miles on the little guy including a 24-hour round trip to New Orleans, as well as minor fender bender requiring me to replace the hood, grille, core support, and front bumper.
RELIABILITY: At about 60k I started to notice a whine from the transmission that started at about 30 mph and increased in both pitch and volume as you went faster. Changed the oil in the trans, no difference in the noise. Read up on the issue...apparently this transmission has a bearing on the end of the countershaft that doesn't get enough oil, leading to noise and eventually failure. The trans is designed not to be rebuildable, so the only remedy is new transmission. The good news is apparently there's a redesign of the case that supplies more oil to the bearing. The better news is that I'm still under 100k powertrain warranty. So I take it my friendly local Chevy dealer (Bob McCosh) who diagnoses it, agrees that it needs a new transmission, and gets the new trans ordered, installed, and the car back to me in 36 hours. I took it in Monday at 7:30 a.m., they called me at 9:00 and told me they were ordering the trans, and told me I'd have it back Tuesday night. I did not believe them. Tuesday at 5:30 they told me to come pick it up. It drives nice now. ALthough it's a very common issue, GM hasn't recalled and apparently a lot of Cruze and Sonic owners get the runaround if they try to take it in for warranty work.
-Before I took the car in for the trans replacement, the shift quality had degraded from "pretty good for a cable shifted FWD car" to "absolute garbage". If cold, it would crunch on the first 1-2 shift of the day no matter how gingerly I shifted, how long I waited while double clutching, nothing. It got a little better when it warmed up. Apparently this is also very common with these cars, and there's a TSB for adjusting the shift cables that will generally cure the problem. If you're out of warranty, it's not a difficult procedure. Shifter adjustment
The car does have a persistent CEL for an evaporative emissions leak. It's the evap solenoid - common failure, cheap part ($45), but it requires dropping the fuel tank. Haven't done it yet. Doesn't affect driveability, and coming from a 90's BMW I'm used to having the CEL on.
-GAS MILEAGE: Not as good as the EPA would predict (39 hwy), and consistently a few mpg less than what the onboard computer reports, but still gets 33-34 on the highway with the air on. Can't complain too much.
I'm out of powertrain warranty in July, and I think I'm going to go with a Bad News Racing tune. Conservatively, it can add 35-40 lb/ft of torque and 25-30 whp without any big downsides, and with no big hits to powertrain longevity or fuel mileage.
Overall, I still say it's a good buy. As a GM economy car, they are getting stupid cheap now. I would still recommend it to anyone needing a high-mpg DD. If you buy one and the trans does go out, a brand-new trans from GM is about $1,700 list.