My ST was brand new, and had apparently been built in June 06, when i got it in July 06 i clayed it as i wasnt overly happy with the finish, you would be surprised at the amount of contaminants in the paint.
Personally I sonus green, or grey if the car is really bad, problem with grey is that it tends to marr the paint work so you definatly need to polish it afterwoods, sometimes even a machine polish is required.
Megs clay is just fine too.