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I have a PowerMac G4 400 and upgraded to a 1 Mhz still running at the office doing graphics, database, and email. It was my first Mac in college in 1999. It runs 10.3.9 and works great. We do simple ads in Photoshop and Illustrator. We'll probably use it until it stops running.

I also have a PowerBook G4 1.25 Ghz since 2002. I changed the battery once. It still runs great. I do everything on this laptop from personal to work. The screen seems to be dimmer than it used to be. And I've worn out the keys A, D, and N. Which happens to spell the first three letters of my name and the word "and" ... I guess I type those letters a lot. I'll probably use this for another 1-2 yrs before getting a different laptop as my primary.

The nice thing I love about OS X is that it just feels new and fresh regardless if it's a 1999 PowerMac.
 
We still have a working Apple II, its lasted quite a while! :)

As for still useful computers, I just got our old G3 Powermac up and running again and its working fine for word processing, file sharing, print sharing, etc.

But as others have said, it depends on what you mean by "last" :D
 
Tibook still running just need new hinges, side fan and if i feel like it, a new case. It does everything a college student needs, Safari, Office, Mail, Adium and iTunes.
 
It really depends what you want to do on your computer which determines how long it will last.

Word proccessing and surfing the net you don't need the fastest or lastest computer for it, however if you're gaming or using highend multimedia application expect to update every 2 years (Moore's Law).

I expect to get 3 years out of my new iMac.
 
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