Wow, I didn't know there was this much debate over 17 inch to 18 inch. I was actually thinking of going from 17s to 19s, but not after reading through this thread. The look of 19s on a 3 series is sickening, but I definitely hear you guys on practicality, performance, risk and cost.
I'm in the market to buy new tires which makes me think just like dpnguyen; get bigger better looking wheels for the summer and use the stock 17s for winter only. I wish I wasn't so indecisive!!!