First of all, the present 330Ci is an E46, not an E90, which means its a 6 year old car, nearing BMW's 7-year redesign. The E90 330i is faster then the 330Ci right now because of the 255hp engine v. the older 225hp engine. So really you compared the wrong cars to start with.
Second, yes, I have driven the IS350, as well as almost every BMW sold in the US since the late 80's.
Ill give you the fact that the IS350 is a very fast car. I never said it wasn't. The IS350 really is the biggest hope for Lexus' future, because it has something that sets itself apart from the crowd as far as driver experience. That 307hp engine is great in all respects. In a straight line, you can't beat it in a luxury sedan unless you buy something much more expensive. To go with that amazing engine is the fit and finish you expect from every Lexus, not to mention the reliability. All of that makes the IS350 a very solid car, probably my favorite Lexus ever.
Now the problems-
1) I hate the transmission. The BMW 3-series, Infiniti G35, Audi A4, Acura TL, and Cadillac CTS all offer manual transmissions. Even the Benz C350 can come with a manual, and Mercedes loves their automatics. For some reason Lexus doesn't offer one on the IS350 at all. In an attempt to make up for this, they gave you those nice little paddle shifters, except they were still attatched to an automatic. Now they get to tell customers that don't know better, "look, we have paddle shifters just like the BMW M-cars, the Audi cars with DSG, and the Ferraris." Here is my equation...
Paddle Shifters + Sequential transmission = F1 Transmission = cool
Paddle Shifters + Automatic transmission = F1 poser = not cool
So not only do they not offer a manual, they try to make the automatic transmission look better then it really is.
The L in Lexus is for
2) Its handling isn't very great either. It just doesn't feel as good as the 3-series. C&D pulled .85 on the skidpad, compared to .89 on the BMW. You can't turn off traction control. Its a little twitchy. Plainly, the steering felt fake; less sports car and more Camry. Hundreds of automotive journalists say about the same thing, so sorry for the cliche, but the BMW 3-series feels like an extention of your body. The car does exactly what you tell it to do, and I like that feeling alot from a sports sedan.
3) This really isn't a biggie for me, but it is a problem for sedan fans. You buy a sedan over a coupe because it has bigger, more usable back seats. The IS350's back seats are tiny, the smallest in the class. So the comfort factor that I usually say is the best feature in any Lexus, doesn't apply to the back seats of the IS350.
On paper the IS350 should be the better car, but it isn't. The acceleration and power numbers are best in its class, by alot. When the IS350 owner compares his car to the 330i, he gets to talk about his 307hp 3.5l V6, the Lexus quality of fit and finish, their reliability, their paddle shifters..., and their 1/4 mile time. The BMW owner only really needs to reply, "but I get a bigger smile when I drive my car," and that makes all the difference in the world to me. You buy the Lexus to impress your passangers, your friends, and the average Joe driving next to you. You buy the BMW so that it can impress you every time you hit the start button, and I would take that feeling over what you get from the Lexus eight days a week.