I know that it varies on dealer to dealer. I also know that companies such as C&C Custom Stainless Hillington near Glasgow Airport regularly have cars in getting them replaced as they have rusted (poor materials) or have broken up inside. Also the tail pipes are often a screw on, Halfords style job. They do not rate them at all.
However, I am sure many people have been happy with a Powerflow system.
The Cooper S may become boomy with too loud an exhaust for motorway etc if badly designed.
Good luck - there will be no scooby rumble though
Brian