We only just bought our HHR after trading in our Chrysler Town and Country Van which I was very attached to, but we no longer need a vehicle that size. I love the HHR, it is so comfortable and fun to drive. I love the interior and the way all the gauges are set up. I was going to go with a tiny car for better gas mileage but the HHR gives me a bit more room with a better gas mileage than my van.
I love the old body style and all the interior controls are very convenient once you get used to them. I really love that I have the buttons on the steering wheel to adjust sound and tracks for the Stereo. It is also very convenient that my back seats fold down in case I need to haul something.
i can think of no improvements at this time but I just bought mine so maybe I will think of something later on, for now I love my HHR.