It seems to be GM's answer to the PT Cruiser. I know they're FWD, and probably share drivetrain components with similar-sized vehicles in the GM lineup, aka Chevy Cobalt, Pontiac G5, and Saturn Ion. That means parts should be fairly common.
Supposedly, the designer of the PT Cruiser defected from Chrysler to GM, where he was placed in charge of the HHR design.
You get your choice of either a 2.2L or 2.4L engine, so gas mileage should be pretty good, although the HHR is probably heavier than the Cobalt, G5, or Ion. For 2008, there's going to be a turbocharged version called the HHR SS, with 260 ponies to the front wheels.
Looks like the station wagon is back again.