have a R56 Cooper but I've drove the R56 Cooper S a couple times when I was test driving cars or simply waiting at the dealership for service (the Mini dealer doesn't care if you ask to take something for a spin). The Cooper S has a little bit of torque steer but it's not unruly like some other hot hatches. The handling is awesome though, I have honestly never driven another car that offers the same amount of fun as well as grip. Granted it's no Lotus, but for a small hatchback it is great.
Anyone who says they are underpowered has no idea what they are talking about, the car is very light and moves quite nicely with the 178hp. The new turbocharged engine is fairly smooth with the 6spd manual, but the 6spd auto on the S isn't all that great considering it is better suited for the base Cooper. But if 178hp isn't enough for you then there is always the John Cooper Works parts that can be installed and warranted by the dealer. They do everything from engine upgrades to suspension to brakes.
They have decent fuel mileage as well and they are surprisingly roomy inside. Like I said I don't have the S but I do have a Cooper and can answer questions you might have. Are you thinking about a new R56 or and older R50 one?