2006 MBP 2.33 Ghz 256 MB 4 GB 42 months - this one has been used and abused - replaced battery once, no different than replacing the battery or brakes on a car
2007 MB 2.0 Ghz GMA 950 2 GB 36 months - not used as hard, but I'd guess 4 hrs a day avg - original battery that stays plugged into the mains 98% of the time and has been sitting at 105% of original capacity for the past year and lasts for 4 hrs when I borrow it from the wife - has been calibrated 1 time
Neither one of them are ever shut down except as required by updates. Software issues on the MBP, but I'll try some stuff that I'm pretty sure won't work before I start. Haven't heard a single peep out of the wife on her MB, unlike the every 3-6 month time frame of her win machines slowing to a crawl with all the spyware/malware she had managed to find and the inevitable once a year clean install I had to do because it was just easier than cleaning by that time.
Aside from just wanting a new machine, neither one of them have ever acted in such a way that has made me think about the need of a new machine while using them. Something I can say about only 1 of the other 40-50 computers that have been a part of my home collection over the last 29 years.