Long version: I had a business trip to DC and Charleston this week and had a chance to rent and drive a Cobalt while in DC. I had the 4-door model without many options. The car was very nimble and had a much tighter turning radius than my Nissan Altima FredMobile. It also had more power. It accelerated onto the busy freeway quickly and without fuss. The car was well-built and I didn't notice any manufacturing defects or rattles.
The only thing I didn't like was the headrest, but I will admit I never took the time to adjust it. As far as the rest of the interior goes, I'm 6' tall and found the car very comfortable. I didn't get in the back seat, but it held all the gear I threw back there, so I imagine it was relatively comfortable, too. If my son didn't already have a car, I would seriously consider buying him a Cobalt.
Aside: I tweeted my approval of this car and within 24 hours, the GM Blogs twitterer had replied on Twitter and thanked me. Now they're following me. Way cool! That's a shows marketing sophistication on the part of GM.