In more than two decades and well over a hundred HDDs (lost count years ago) I recall only four drive failures (Maxtor, Seagate, WD, and a Tosh 2.5-incher). My hardest working drives have been IBM/Hitachis, followed more recently by 2TB WDs. I still have about 70 working drives lying around gathering dust.
A lot depends on how you treat 'em. Taps, knocks and bumps when powered on can be calamitous, but the biggest killer is thermal cycling, which is why I keep all main systems running 24x7. I still shake my head in astonishment at the incredible capacities, low price points and amazing reliability of today's drives. Still, I'm obessive-compulsive about backups ... the reason I'm online right now is because I'm killing time while doing a weekly backup to two external 2TB drives, the fourth backup this week of the same stuff to different drives/locations. It's only in the past 5 years or so that having enough disk space hasn't been the Big Problem it used to be in the early days.