I've owned my 1999 convertible Mustang for 5 years with only 1 problem. I had a problem shortly after I bought it with one of the convertible top hydraulics going out. The car was used, with 17,000 miles on it, before I got it and apparently someone tried taking the top down with it still latched into the windshield and this caused the failure. Other than that I have had no mechanical problems with this car and it is used as a daily driver. The Mustang is a little bumpy because of the sports suspension and height from the ground but that's what you come to expect in a performance RWD car. Don't expect to be riding like a Caddy in this bad boy, but you'll ride in style!
Power for the dollar, convertible top for those summer days, it looks hot, mechanical reliability, good on gas.
Body flex, though this has been addressed in the 2000 - 2004 Mustangs. Rear seating is cramped but realistically this is a 2 seater cruiser and not a family car.