My friend had in total 3, Cooper S, Cooper, and now facelift 1.6 diesel. He swears by them, in terms of quality and especially road manners. One thing to watch is gearbox on early (-04), only Cooper with Midland box, S is different (Getrag) and strong.
General quality is OK, but you have to be aware that this is a little car, and little cars aren't built as tough as big ones.