If you're using the HS with an AMD processor (eg., a T-bird Athlon or Duron), then the pad is what is generally recommended by AMD (it's a phase change thermal interface compound and it helps to cushion the fragile die against the metal HS). The downside of the pad is that if you plan to frequently remove and replace the HS, then the pad will pretty much be useless after removing the HS that first time (PCTC is a single use product). At that point, you should remove the pad and use good quality thermal grease such as arctic silver - but placed only on the CPU die, and spread in a thin layer.