It really depends on who you're buying it from and who inspected it. Wikipedia sums it up pretty well.
- The factory basically makes it refurbished but the price is generally higher.
- The dealer might certify the car themselves which means it's up to them how thorough they are.
- An independent agency might be used that could be less biased but still set their own standards.
They all generally come with warranties that you can't get on a used car, which affords some peace of mind. A second opinion from a mechanic would still help you know what you're getting into and if it's what you want.