My early 2008 Core2 Duo w/4gb ram is still going strong (more or less). It was really slow under 10.8.x, but 10.9.3 sped it up and it feels like it got a couple of years younger. That being said, it just doesn't have enough ram to be comfortable, and despite the fact that it runs well, it takes it's own sweet time launching apps, and even the "Save" dialog box can take 5-10 seconds to appear, although Default Folder is responsible for some of that delay.
The OEM 250gb drive died in it's 3rd year, and Applecare replaced it with a 500gb at no extra charge. It's started to run hot lately, and I have a clip-on fan behind it to help keep it cool.
I'm at the point where I'm ready to buy a new computer if the heat problem gets any worse, but I'll keep chugging away with it until the next real refresh if I can. I'm hoping they'll bring the price of the SSD down in the next gen, and if they do, a few more months with this one will be well worth it.