Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_3_3 like Mac OS X; en-us) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8J2 Safari/6533.18.5)
For me it's the intigration of hardware and software. Apple designs the product and the software that runs on it. I also like the fact they control (lock down) the device. I seldom worry about viruses/bugs/etc. It just works.
The hardware is a source of pride for Apple. They really go out of their way to provide eloquently feeling devices. They have the money and resources to provide top notch products at less than what can be provided by the competition.
Finally, the ecosystem. As a personal user, I use my iPhone mainly for the experience. I dont require exchange service through work. I can check my email, surf the web, share contacts, etc effortlessly. The investment in music, movies, apps, etc also keep me squarely on the Apple side.
It doesn't hurt I use Apple computers and have iPads as well.