I had an 06 cooper S for about a year and a half, no major problems. Got it at about 60k miles, put 10 or 15 on it. There's an airbag sensor under the driver's chair that can trip the warning light if you move the driver's chair just so while the car's running - that was a PITA. Had to replace a tail lamp - a nightmare unless you have double jointed arms the size of a 10-year-old's. Don't buy an automatic up to (I think) 2007 - those are the ones prone to failure - $2000 replacement.
But ours was a ton of fun, for the price and size of it. It's no Porsche, by any means, but the added hp and better suspension with the S was great. Lots of torque steer that took some getting used to, and it definitely handles funny at the track because it's FWD - tons of understeer, but still a fun car for a small track. A lot of fun to drive spiritedly around town, especially with the top down. Trunk space isn't huge, but adequate, though cut it in half on a convertible. On a coupe, with the back seats folded down, I can get my entire guitar rig in there - 4x12 speaker cabinet, head, 2 guitars, a case of gear and a duffle bag of cables, and still have room for a passenger in the front.
Gas mileage is good if you drive it slowly, but you won't.
We got consistent mid-20s mpg because I loved to drive it hard.
The only thing I absolutely hated about the car was the electronic throttle. I'm used to the 911 linkage to the carburetors, where the most minute change in pedal causes an instant reaction in the engine. In the Cooper, you hit the pedal and wait. I installed a sprint booster (throttle remapper) which helped, but only by about 50%. That took it from unbearable to bearable, but still not great.