Sports Car International (March issue) did a comparison between the G35, IS350, C350 and 330i. They ranked in the listed order, from bottom up.
As you noted, the IS350 looks really good on paper and this is stated in the review:
"In purely rational terms there is a strong case to put the Lexus in second place, or even to declare it as the overall victor"
The knock against the Lexus is the same as always:
"...too much pure, unfiltered feedback has been dialed out in the quest for perfect road manners, which ultimately makes the Lexus too distant and unengaging to be truly inspirational."
"...the IS350 has more horsepower and equipment than the others, but those advantages just don't make up for the lack of character."
This sums up the review for the 330i:
"...the simple truth is that there still is no better sport sedan for the dedicated driving enthusiast than the BMW 330i. You could own this car for years, giving it a good thrashing on your local track every few months, and probably still never encounter the absolute limits of its ability."
Is it just a case of the SCI journalists not daring to stray from the pack? I don't know. I've driven the E90 330i but not the IS350 so I'll hold my opinion for now. I don't like that fact that you can't get a manual in the IS350 (yet).