I like the fact that I earn most of my miles for business-related travel and then redeem them for vacations.

There's no high in the world for me like sitting in a business- or first-class seat on an international trip and thinking "This would have cost me *thousands* of dollars if I had to pay for it out of my pocket.

The one catch that I've found is that I've become so addicted to requalifying for elite levels each year that I no longer redeem miles for a free vacation ticket. I always buy the cheapest available upgradable ticket and use the miles for an upgrade.