Ironically I just did a review of the GS450h today:
and here's a review done between the IS350 and Q50 so you get an idea of how the car drives
The IS and GS are geared towards two different demographics, but one thing they share in common this they are both the Ultimate Driving Machines now dethroning the 3-Series and 5-Series respectively as the best handling cars in their segments.
I like the new IS but to me there are too many compromises to the Luxury feel which took a backseat to Sport. The GS on the contrary blends Sport and Luxury in harmony. Unfortunately though the technology aka Navi is already dated vs the IS.