I have had the IS250 this week while my IS300 was in for warranty work. The IS250 looks the business, is incredibly quiet and smooth, has great comfort features and toys, has improved fuel economy, and performs similar to the 300..... and yet, I enjoy driving my IS300 far more. I have no pretentions to be a Jeremy Clarkson, and I struggle to explain why I prefer the 300, I just know I do.
The only thing I can put it down to is the old model is more involving. I am guessing now because I have never driven the GS, but it seems like the new IS is just a small GS with firm suspension. This therefore falls somewhere between being an out-and-out luxury car and a sports saloon, without quite achieving either.
I would love to hear other opinions because I had the IS250 mapped out as my next car. Maybe I didn't give it enought time.