24" iMac 8,1, 2.8 GHz Intel Core 2 Duo, ATI Radeon HD 2600 Pro 256 MB. Bought in May 2008. Still a sweet machine, does what I need. The only real problem was the optical drive failed in 2011 but happily before AC expired, and the replacement drive has been flawless.
I've installed more RAM twice: from 2 to 4GB in 2009, and then to 6GB yesterday.
In June 2012, I had a local Apple-certified Mac shop replace the 500GB drive with a 1TB, do a clean install of ML, and of course clean the dust from the innards. The original HDD wasn't acting cranky but it was getting very full thanks to all my photo and audio files. I've been pleased with the bigger HDD, and I think it helped that I "started over" doing clean installs of apps etc.
I don't know if I'll buy another iMac when I'm ready to give up this one (or it dies on me). I'll probably get a mini, or one of the MBs. I hope I don't need to make that decision for another couple of years.