I still have/use my late 2007 / early 2008 24" 2.4GHz, 4Gig RAM model.
The original disk crashed after 6 month of usage, i got it replaced by warranty. Then after just about 2 years i started to experience graphic clitches. It was the Ati HD 2600 chip which was failing. I got that replaced also by warranty.
Later i experienced yet another hard disks crash(good thing i have 100% backups). This time it was way too late and too slow to take it to a repair, so i replaced the disk by my self. I have opened my imac since then 2 times to clean the fans. That reminded me, i should do it again soon... I have two cats in the house
Currently it runs just fine, albeit little slow (hard disk is the biggest bottleneck) because i have it fully encrypted with filevault 2 with lion.
I use it in everyday general internet stuff, sometimes movie encoding(final cut), photos(aperture), iphone syncing, PLEX as movie server outputting to projector and so on...its not fast by anymeans, but it gets the job done. It enough for me to watch my 1080p / 720p movies without dropped frames...
I'm going to update in couple months and waiting for the imac refresh. I can then give this imac to my girlfriend so she can play her games which run just fine under bootcamp. She's not complaining about FPS as she plays some MMORPG games and isnt nitpicking about GFX performance.