I use to have 18s on my old Focus. They look sweet and fill in the car nicely however I don't recommend them. They just aren't reliable enough when you're on 215/35. After having 17s on my SVT I can say that 17s are perfect.
I like the look of 18s on compact cars. The bigger the wheel the better on any car, but for practicallity sake, I'm putting 17s on my car. I'd like better ride quality, and I'm gonna drop the chasis down a few inches anyway. I do love that wheel well full of wheel though!!