This is my first foray into this sort of car. I researched for months and found myself really turned off on the 330. I really like gadgets, and the luxurious interior of the IS sold it for me.
I picked my new 350 up two days ago and, I can assure you, driving your own is 1000X more exciting than a test drive. I'm sure this holds true for the BMW as well.
There are some questions about how an IS handles vs a 330. I found that the sports package resolved those issues for me.
I have a friend who is anti-gadget. He needs 4 tires, a steering wheel and a radio. This makes him a BMW sort of guy. We were going to Thai for lunch yesterday. I asked him where it is and he said just go that way. I hit the talk button and said "Nearest Thai Restaurant". He immediately said "NO WAY!". Sure enough, the voice came back "Finding nearest Thai restaurant." Sure enough, it found Chadra, in San Ramon, CA, exactly where my buddy was taking us. That is just a miniscule sample of the gadgets this car has to offer.
This is a tough place to get this sort of advice. You'll get an opposite response from the BMW forum. What I can tell you is that the IS is a remarkable car and you won't be sorry if that ends up being your decision.