Except when the HDD crashes
This is why I don't have an iMac.
Instead I have a proper desk top with multiple HDD's.
I have two holding my photos, a primary and copy.
If the primary fails I simply start using the copy and lose nothing, then replace the failed drive and use that as a copy after doing a back up.
If the drive with my OS on it fails I simply plug in an older smaller drive that already has a simple system install on it, and away I go again.
If something major fails, I simply take out one of my photo drives (or one of my externals) and plug it into my GF's computer, and away I go again.
Down time would be measured in minutes, not days
Of course if your running proper external back ups, and you can live with the down time, then it's not as much of a problem.
I just upgraded to an iMac i7 27" and this is exactly my concern. My PC has 3 drives in it and an external attached which is easy enough to somewhat duplicate with the iMac. But that internal system drive is something I don't think I can change myself. And I don't even mind the downtime. What I don't like is having to PAY someone else to replace an HD.
Maybe I'll get that 3 year extended warranty.....