John Gruber, of DaringFireball fame, just had his HD fail. He noticed it because everything was running very slow. He ran DiskWarrior which recovered his data but alerted him to a problem. SMART Utilility would most likely have alerted him to this issue a lot sooner. He was lucky because he also had a SuperDuper backup as well as a DropBox backup.
He makes a lot of good recommenations, and I encourage you to read the full story, but this is my favorite part:
Hard drives are fragile. Read as much as you can bear to about how they work, how incredibly precisely they must operate in order to cram so many bits onto such small disks. It’s a miracle to me that they work at all. Every hard drive in the world will eventually fail. Assume that yours are all on the cusp of failure at all times. It’s good to be spooked about how long your hard drives will last.
A hard drive failure can happen to anybody. Be prepared when it does, and be alerted before it does. That’s why I made SMART Utility.