The life of a computer can vary I think. But on the whole Macs are ok, however, they have certainly had their issues, the iMac yellow tint screens is one.
I have had three Macs now, I had the non unibody MB Pro which was my first machine, the power board failed in it and had to be replaced, known issue, and when it was done the case was slightly bent on the piece going across the top of the DVD loading slot.
Not impressed.
I also hated the keyboard on that model? Really couldn't get on with it for some reason, anyway that went the moment the unibody machines were launched with the chick let keyboard which I can use fine.
But I had to go through two machines before getting a third one with no thumb prints behind the glass! Not impressed, also i think the DVD drive was packing up on it.
Anyway I then traded to my 2010 Mac, it has been fine since day one, solid as anything, I have managed to drop it off a bed onto carpet and it was mark less! No damage at all, it has some light scratches on it and some light scuff marks from general use, but it feels as solid as anything!
I looked a Retina Mac and loved it, thought about getting it to replace my current one but then also thought why? It's a 'want' and certainly not a 'need'. So I'm sticking with my 2010, and putting an SSD in it next week, along with my 8GB ram and Mountain Lion I am set. It runs Diablo 3 like a dream, better then Starcraft 2 I think! I really cannot see it not lasting for a very long time and if I replace it, it will only be because the temptation for new tech has become too much, not because it needs replacing.
But on the flip side I have an Apple Macintosh Performa 460 that STILL works, I think it's about 17 years old? Says it all really.