Stating the obvious, after having mine for five years -
The GS is a totally different drive. Softer suspension than the IS and loads of interior space. Fuel consumption is wicked around town and 30 if you are careful on the motorway. It's not a sports car but at 8.6 to 60 it does not hang about and the brakes and handling more than match up. Very smooth and quiet but not quite as much as an LS.
Mine's the SE model with sun roof, leather and satnav. The leather which they use is absolute top notch, far better than Jag or Mercedes but the non-SE model has Alcantara - an Italian man-made suede which is really nice.
Sound system, heating and aircon are all first-rate. Engine and gearbox problems would be very unusual.
Satnav models will always command a higher price but I use a portable TomTom in other cars and despite what everybody says, is every bit as good as the early Lexus CD-based system.
Don't kid yourself that GS's never go wrong. Ball joints, front bearings, central locking all fail and reckon on a set of front disks for every two sets of pads. Bearings are serious money when you include fitting and realignment after the hubs are refitted. But then if you want total reliability, you would be looking for a Honda.
Never buy a GS without all the stamps in the book. They are at bargain prices outside the Lexus network but if you are looking to buy from a dealer, negotiate the longest warranty you can.