The car is brilliant. When price is taken into account, it's the best car in the world, objectively speaking. That's not a dramatic statement. An examination of everything this car is good at, a comparison to competitors and of course, a consideration of value, will result in this judgment.
The engine cannot be ignored. It's not the best V8 in the world, but it's the best one underneath the Corvette (which is 45k--15k more than the base G8 GT). It has power, everywhere. My other car is a Subaru WRX, so I've been used to a peaky powerband, and the combination of the 6 speed automatic transmission with an engine with an amazingly flexible powerband means that at any point you can have huge power. The acceleration from a dead stop is very good, but the high speed acceleration is phenomenal. There are cars even in its price range that can out accelerate (citing, say, an STi) from a stop, but from a 100 on, the car pulls hard.
The fuel efficiency is phenomenal as well. It's rated at 18/24, as I recall, but I routinely get 30mpg on the highway (although, in fairness, the town is about the rating--it's hard not to keep it in 1st or 2nd to feel the power).
The handling is terrific--and don't get me wrong, I'm not saying you're going to turn top lap times in your G8--but it's terrifically fun. The car just handles so brilliantly at 9/10ths. It lets go of the grip gently, and in a balanced fashion--it's an excellent drift car. The engine and transmission are flexible enough to let you steer with the throttle during a drift at any speed.
The interior is quite nice. A moderate overuse of plastic, which is common in luxury cars under 50,000 (having tested IS350s, G35s and 335is rather extensively before this purchase), is present. The large red display in the middle of the console annoys many, but I appreciate little quirks. The gauge cluster is adequate, but the gauges, like in most cars, are in the wrong places. The speedometer or tachometer should be the middle of the cluster (depending on whether or not the car is an automatic/smg or manual), but instead, the speedometer and tachometer are on the left and right, while a helpful, though misplaced, data display is in the middle--that's okay, but that's not performance minded--go check out a Ferrari or Porsche gauge cluster to see it done right.
The manuamatic/tiptronic shift area is reversed. It's more intuitive, in the real world, to put upshift closer to the driver and downshift farther away.
The front bumper is too low for the real world. There's no paint left on the bottom of mine.
Wind noise is too loud--not more than an ordinary car, but this is a grand tourer, and it ought to be quieter.
Ride is nice, about where it should be for this segment.
Tons of room on the interior and trunk.
I love the heated seats, fully auto dual zone climate control, LCD display, onstar and remote ignition.
I could probably write a lot more, but this should be an adequate review for most potential G8 buyers.