I went from the IS200 Sport to a 99 model GS300 Sport. It was the perfect combination for me as I wanted the looks and needed the size. It's my version of a sensible family car!
I thought I'd find the GS harder to park than the IS, but it's amazing how quickly you get used to the size, and it's really never been a problem. It provides a great drive although as has been said, if you want to fling around corners, it won't be the car for you.
The majority of my driving is in-town which pushes the petrol consumption up, but I find it worth it as I like the comfort relating to an auto. On the odd-long haul motorway drives I do, I get out the other side still feeling refreshed as if I've had a quick drive to the shops.
I think the majority of the plus points have been covered but for me now, the GS wins over the IS hands down. It's a good combination of luxury and looks and suits my family needs (although the sport being slightly lower can be a pain in the bum - I grounded my car on my grandparent's steep drive and the car is now relegated to the road when I visit them!)