Mac computers definitely last longer than any other brand of computer for a few very simple reasons: 1) You will be up to speed much faster on a Mac, because they are so intuitive and easy to use. So for every day you will use your computer, you'll get lots more done, with less stress & downtime; 2) Being part of a more closed system, Macs are not subjected to the viruses of PCs, which also translates to more uptime/less downtime; 3) Macs often alert you to free downloads of continual upgrades, making your machine faster, more efficient, and less troublesome all the time. You get more mileage in less time.
Many people point to the $500 to $1000 premium a Mac might demand, yet would think nothing of dropping that same amount each year for, say, a higher-def TV or a better cell phone package. The way I look at it, having a superfast, easy to use and trouble free computer is well worth the cost of less than one frappé every other day.
I've tried IBMs, Dells, Compaqs, Sonys, HPs, Gateways and Toshibas, and would never consider anything other than a Mac ever again.