nsap, like you I had my concerns about buying the G8, especially in the GT variant with where gas prices are right now. I preface what I am about to say with the caveat that I am NOT an automtive engineer. While I consider myself a sort of car guy, I by no means I think I am the Oracle of all things automtive. In helping make my decision:
1) I channeled my system analyst and created a fuel cost analysis of similar vehicles with large V6s. Included in the analysis were the Infiniti G35, 2009 Maxima, Lexus IS350, and TL Type-S and a couple of SUVs (GMC Acadia and Ford Edge). For controls I threw in an Accord V6. The MPG delta between these vehicles and the G8 GT was not that significant. City driving is what hurts the numbers, but most of my commute is highway where the cylinder deactivation helps tremendously.
2) The G8 does not require premium fuel, whereas most of the aforementioned vehicles require premium fuel. Living in San Diego, this is a HUGE plus for me as in some parts the fuel still to this day, is north of $5 for premium in some parts of SD. By the way, the foreign/'premium' nature of some of the aforementioned sedans made them more expensive to own, projecting out for insurance as well.
3) Sheer comfort. At 6'2 and on the big dog size, I have tried prying my girth into many different vehicles looking for a comfortable fit. Not only is the G8 a great fit for me, but also for my passengers. One prime laughable example of a poor fit was me trying to fit in the Lexus IS. Even with the driver's seat parked way past the b-pillar, there was no way I could drive that car comfortably.
My .02...ahem.... $.03