As an ISP I currently run about 50 harddrives, these are SCSI and properly cooled. In ten years I have had exactly two drive failures - both Quantum. I've only got two Quantums and they both failed in less than a year. Needless to say I don't buy Quantums anymore. The rest of the drives are either IBM (now Hitachi) or Fujitsu. I switched to SCSI around 10 years ago because of a couple nightmare situations due to failed IDE drives. Several of the drives I'm running are approaching 10 years and the average age is probably around 5 years.
OTOH I've had very bad luck in workstations with ATA drives, particularly Maxtor. Average life seems to be about two years, sometimes much less, occasionally somewhat more, but then most PC boxes don't provide adequate cooling. IMO IDE drives have no place in a critical application although I've heard that some WD drives are built to commercial specs.
Even 50 harddrives is just a widdle in the ocean, and must only count as anecdotal observations, at least compared to the studies cited earlier in this thread.
And if you note NetApp's observations regarding reliability, you'll find that they started using ATA drives around the turn of the century (it feels funny putting it that way). They were not the only ones.
Your personal experiences may be due to that earlier, SCSI drives used 10% of their capacity for automatic reallocation of bad blocks. Where an ATA drive would use the full 40 GB capacity, for instance, the corresponding SCSI drive would use only 36. When bit rot set in, the SCSI drive had quite a bit of headroom, while the ATA drives didn't.
I'm not sure whether current SCSI drives still make use of this feature. It may actually be less helpful today than it was ten years ago.