I had to take the iMac back to the store on Saturday for the third time. It refused to show me any image, no matter what fixes I tried. The display was blank, while everything else worked (I could connect and browse the hard drive). I did some research on the web, and apparently that particular problem is only "fixable" by getting a new machine.
I mentioned that to the guy who registered the problem, and he said he'll see what the repair people and Apple say. I have no great hopes that they will give me a replacement machine that works. At least I shall request my money back for the repairs that have not repaired anything. That, or a discount on a new iMac for the same amount.
I am very glad that I bought the MacBook Air. Otherwise I'd be without any computer and I remember how horrible that was.