Sandy, he's 100% right.
I just came back from a 2 hour ride each way in my 2007 DTS with the Northstar V8, and pulled 25-28 mpg easily cruising at 70 MPH. Around town? Pfft. Maybe 12? It's just the way they are. I think they've got some great highway gearing, but around town is abysmal.
Even though I miss my Scion xB's 32 highway 26 around town, I wouldn't be willing to go back