To be honest, the state of the road surfaces around here I am cringing a bit at how rough the ride might feel on the 18". My E46 330Ci has the standard wheels (17"?) and my teeth rattle when I hit a bad pothole.
The 19" look fantastic but, apart from the ride, they cost an ADDITIONAL £560! (That's £890 over the standard ones.) I think they deliberately chose ugly wheels as standard - I just don't like them at all!