If you're used to nice, comfy rides of your old BMW's then you're in for a shock if you go for the 18" on runflats! I was surprised how hard the ride was when I got mine a couple months ago. I've had sports cars before & I know the ride is meant to be hard, but this was a shock. An absolutely fantastic car otherwise, but because I do about 2500 miles a month it was spoiling the enjoyment (and no doubt my back!) - I was more concerned about driving around potholes, ridges & grids. Cats eyes on motorways sounded like explosions going off.
.... and then I swapped the fronts to non-runflats. Halleluljah! Transformation or what, and much quieter. I can't wait to get the rears done. I really recommend you budget an extra £400 & get them done as soon as you buy. It's a totally different machine. And don't listen to a word BMW tell you about upsetting the balance etc of the car, absolute b'llox.