Assuming that the problem is wide scale on this particular model is absurd. For every person in a forum who got a "crap" iMac I'm sure there are 100's more people who got a perfectly functional iMac. .......
A number of people reported replacing computers 3 - 6 times, and even that didn't guarantee a problem free computer (and I am talking a DOA kind of problem). This pattern suggests an extreme lack of luck of those individuals, or a high level of faulty units in the batch.
Tom B.