27" iMac, 26 months, no problems ever
24" iMac, 30 months, no problems ever
24" iMac, 42 months, no problems ever
7 Macs in house, none with any problems the past year, but none are really old.
We had two older iMacs, a 17" G5 which failed (power supply capacitors) after 6 1/2 years, and a 20" C2D iMac which had the display fail after 2 1/2 years (covered under Applecare), was given away and recently had a hard drive failure (5 years). A 12" iBook was used for over 4 years before selling it and had no repairs. A 4 year old MacBook is still being used by my daughter and had no repairs beyond water damage death which I had repaired outside of Apple's system (they wanted more then the cost of a new computer for repair, I had it done for about $350).
I can't say if this is particularly good or bad. I've got a Dell Optiplex GX60 desktop that is 9 years old and has never been repaired, and my last decommissioned Windows box, a Dell Dimension 8200 ran 24/7 for over 6 years and never had any repairs.