I'd be more likely to compare the IS350 to the G35, since the IS250 has a smaller engine.
I drove the IS 250 and was sold on the ride, luxury, and handling, but it was too slow. So I bought the IS350 and got all the same plus the power.
Since the local Infiniti and Lexus dealerships are owned by the same person, and at the same location, I was able to also test drive the G35 and the two Lexus models during the same time period, with the same salesman. The G35 comes close, but doesn't have as nice interior, std features, and has a harsher ride. It's probably a matter of personal taste, but I felt the IS350 was superior and worth the price. After driving it 3 months I still feel the same way.