Just my two cents here and this is probably all its worth given I don't own a Genesis yet. However, I have been test driving them numerous times and here is what I have witnessed.
Hyundai clearly needs to have better quality control. A most basic step in my mind should be that every Hyundai should be calibrated thoroughly once arriving to the dealer. On more than one occasion at Hyundai dealers I have had Genesis's with low tire pressure, high tire pressure (+10 psi), alignment completely off where the vehicle veers to the right or the left, tire balance issues (the steering
wheel actually shook while driving and the salesman said it was because the car had been sitting in the grass for an hour?!), and battery issues where the demo car needed to be "jumped" to start. I have seen all this from probably three test drives at three different local dealers.
Now, once I have gotten to drive one that I feel is calibrated properly I have noticed a few things. Again, all this needs to be taken with a grain of salt since I am not "living" with a Genesis yet.
I find a pretty big difference in handling between the V6 and the V8. Naturally, I know there is a weight difference but I don't know whether the suspension is dramatically different between the two. I find the V6 steering to be much more sensitive whereas the V8 tends to feel a bit more substantial if that makes sense. Between the two I like the feel of the V8 more. It just feels more pronounced and seems like it would have less of a tendency to react to what the road might be doing.
The other item that I thought I would mention after reading much of this dialogue is that I think the reason behind the very different reviews may be based on where the driver is "coming from." Some drivers reviewing this car come in expecting to compare it to other luxury cars. Other drivers reviewing this car come from a more value conscious perspective and it is probably a step up from what they are currently used to, accords and altimas come to mind.
Personally, I am in the second group and after driving an Accord for years once I stepped in to a properly calibrated Genesis I continue to drool over the thought of getting one. The Genesis feels stable and responsive to me while maintaing a very solid feel. I almost think of it as a next generation Accord or the Accord 2.0.
Conversely, I have also driven a number of luxury cars recently. This includes mostly the Lexus LS and ES as well as the Mercedes E class. If you start attempting to compare those luxury cars to the Genesis I think your perspective changes dramatically. Those cars do feel stable to me but do exhibit some body motion as you glide down the road. At low speeds those cars absorb any imperfections in the road whereas with the Genesis you do feel them. The Genesis does give you a good feel for the road at many different speeds. Ultimately, I think if you come from those cars and then drive the Genesis it will feel like a rough ride.
Again, just my two cents. YMMV