I've had 3 major issues after only just over 2 year of ownership (luckily I bought the AppleCare extended warranty).
1. HD - 1TB (crashed) and screen issues (yellow tinge) after one month - both replaced.
2. HD - 1TB crashed after approx. 1 year - replaced with a 2 TB because of zero stock and possible 6m wait (after serious haggling)
3. Screen crash - just replaced
The iMac has been in the service center for more time than my car has been off-road in the same period (only routine servicing)!!!!
Believe poor reliability is related to the special build (upgraded processor and ram when initially ordered)
Have discussed with first level of Customer Relations (CR) who insist this is an isolated case. I think I've got a lemon and am concerned about the cost of repairs when the warranty runs. CR are adamant that they cannot extend the warranty.
ANY comments ??