I would stay away from the 2009-2011 iMacs. Apple still hasn't fixed the grey-smudge or yellow screen issues. They have yet to even acknowledge the grey-smudge problem. It is a hardware defect in all of these iMacs. The newer ones seem to be better since Apple re-designed the hardware and properly shield the screen.

I have a Late 2009 iMac 27" - one of the first shipped - with no screen defects. The initial problem with yellowing screens was related to the diffuser coming loose in transport, and Apple has replaced those as requested. Gray smudge I've never even heard about.

Well, I assure you, the screen yellowing problem is still occurring. I've talked with people who went through 2-5 different screen replacements that had the same problems or worse. Mine wasn't as bad as others. It was the typical bottom third yellow tinge, but it wasn't incredibly strong. You have to really run the macrumors grey screen test to see it: http://forums.macrumors.com/showthread.php?t=824940

The grey smudge issue has affected nearly every iMac owner I've come into contact with. Here's the forum if you don't believe me: https://discussions.apple.com/message/21865410?ac_cid=tw123456#21865410 It's a hardware defect on every iMac. It will happen sooner or later.

And apparently just recently, someone posted that even the new thin models are plagued with this issue as well. I tend to think it has something to do with Apple putting hot-running hardware too close to the screen and not leaving much breathing room for the entire system. An LCD repair specialist went on the forum at one point and said it looked just like cell bursting that he has seen in damaged LCDs. The cells literally heat up to the point of bursting and cause what looks like a grey smudges.

Well, "It will happen sooner or later" isn't very helpful - on a long enough time scale the iMac will be destroyed by the expanding sun, but that isn't relevant to any purchasing decision. Mine is now out of warranty, I've used it a lot including long-term software overclocks of the GPU (which REALLY revs the fans, let me tell you) and there are no problems. I don't believe that all iMacs will eventually fail in normal operation.

You are very lucky then. I would keep an eye on it though. Luckily, my late 2009 27" model does not make grey smudge marks that stay there forever. However, when I use CS6 or increase load on the GPU or CPU even slightly, a weird suction/grey-smudge appears on the far right side. If I shut it down and let it cool overnight, it seems to let up. This is a separate issue that the weird grey smudges that happen all over the screen and don't disappear, so mine is certainly nowhere near what others are experiencing.

Within the first month of me owning it, the hard drive failed. Apple just got around to providing a replacement program for defective Seagate hard drives. I wish they'd do the same for their screen issues. Although, they don't seem to care. I spoke with a 3rd party Apple Authorized dealer repair guy, who more than acknowledged all of these issues. And he agreed that there's nothing much anyone can do about it until Apple acknowledges these screen issues or provides a fix.

So, all I'm saying is these iMacs are defective. You may have a machine with no issues, but that doesn't mean a majority of them aren't defective. I am never getting an iMac again. And I recommend others stay away from them as well. You don't have to just believe me, anyone can go read the thousands of complaints in those forums of people who used these in complete pristine environments and cared for them generously.

Right. Which is more likely - that a majority of all iMacs shipped since late 2009 are defective and that this has somehow escaped the attention of consumers (who keep buying them), lawyers (who would like nothing better than a class action lawsuit against a company with deep pockets), the media (which seems to grab every chance it can get to get a bite at the apple lately, and generally everyone else who might be interested

- or -

that you are part of a small minority.

I know which way I'd bet.