My 2009 24" iMac (3.06 GHz Core 2 Duo, 1TB hard drive, 8GB RAM) has actually been nothing but trouble.
*After a couple of years my screen developed large dark 'smudges' around the top two corners of the screen. These eventually spread further into the display. I removed the glass cover and attempted to wipe them off, but clearly the LCD display itself was damaged (either heat damage or dust).
*I bit the bullet and bought/installed a replacement LCD display. Within a couple of months the same problem developed. I know people are going to say it's an environmental issue - my house is smoke free, pet free, and I change my HVAC filters every couple of weeks.
*Shortly after that the iMac started randomly shutting off (no warning, just loss of power). I initially thought it might be a heat issue but I installed an app to monitor temperature and all the readings were normal. In addition, in some cases it would run for days and other times it would shut off within 10 seconds of booting.
And how did I punish Apple for selling me such a defective piece of crap? By buying a brand new loaded late-2012 27" iMac of course!
I'm now mulling over whether to shell out $150 for a new power supply for the old iMac to see if that fixes the problem. Throwing more good money after bad.