I took a spin in the Cooper S yesterday and to be honest its also a great car to drive. The driving position is excellent, but the supercharged engine seems very smooth compared to the 172. Its nippy but not as raw.
I reckon it would be a great car to drive, probaly as much fun as the 172 but its damn expensive. Its 27K (Euro)here (Dublin) and thats basic. If you want the standard stuff that comes with the 172 add another 6K.
Its also damn small in the back. But it has a 2 year warranty and is BMW reliable.