I got tired of the stock bottle-cap rims and wheels. I happened to find a beautiful set of like new tires on a set of 18" chrome rims, that were somewhat conservative e. They came off an e36 and the price was really right...so...!
When I first put them on, I loved the rims and the 225/40-18 tires, but thought (as others have said) 18" was a bit much. They have grown on me, plus I have gotten so many positive comments from other BMW drivers that I am totally adjusted to it.
I do not have any rubbing problem or turning problems though diameter is increased by about 3/4" and width by about 1/2", about 2 MPG increase in actual speed over what speedo says.