I have done quite a few modifications to my car over the past two years of ownership, but nothing makes it really unique and custom like some new wheels. Fortunately, a good friend of mine whom I originally met on the E90Post forums is in the process of buying a new M3 coupe and he was selling his 19" BBS-CH wheel and tire set from his 2006 BMW 330i.
I had the set installed yesterday and had to add TPMS sensors since the 2006 models didn't need them. This added some minor cost both for parts and installation. It seems worth it since you have to get the dealership to program turning off the warning light when no sensors are installed. Seemed like an annoyance I wanted to avoid so I went with new TPMS sensors. Tire Rack has a great non-OEM set that ran $200.