I had an IS200 Sport and then swapped it for a '07 GS450h SE-L just over a month ago. The GS is back at Lexus for some warranty work, and for the last week or so, I have been driving around in a '14 IS250 Premier and have clocked almost 1000 miles on it.
The following is my quick opinion after driving all three Lexuses for relatively extended periods:
Interior quality - IS200 feels like it has the most "solid" interior of the three. After 13 years and 130k miles, it is still creak and rattle-free. The GS on the other hand has a mixture of creaks and rattles depending on the ambient temperature and road conditions. The IS250 loaner has 2000 miles on the clock yet it does rattle slightly on certain coarse roads.
Ride quality - GS wins hands down. Very smooth ride and excellent seats. I find the IS250 to be rather firm (not uncomfortably so) and the IS200 is somewhere in between.
Handling - IS250 stands out here, it does feel like the successor that the IS200 would be proud of :). Very sharp car and I really enjoy driving it at the moment.
Audio quality - This baffles me. Both the GS and IS250 are fitted with Mark Levinson systems. Yet, to my ears, the 8-speaker system on the IS200 Sport still sounds the best! With all presets at neutral, the GS sounds bass-heavy whilst the IS250 is treble-heavy. To me, the IS200 has the most "natural" sound of the three.
That's my take on how the IS200 stacks up against those cars. :)