Ive had some bad experiences with mild steel in the past. I bought a Jetex back box that was meant to have all sort of super coatings on, it holed in 3 years. One of my friends had another english brand, that last 1.5 years he managed to get 3 out of it but by the time he sold the car it was more putty and patches then metal. I have heard people having similar experiences with HKS mild steel exhausts. Maybe apexi are better as ive not had any bad storys of holed exhausts.