How it looks: Terrific. Everything from the sophisticated typography to the smooth animation effects contributes to the richest, most attractive environment ever put on a handheld device.

Built-in applications: What's good is great--especially the Safari browser, which makes navigating around sites that were never designed to be viewed on a phone remarkably simple. And the OS's music and video programs truly are of iPod caliber. But as a productivity tool, the iPhone lacks depth: You can't search e-mail, and you get no apps for editing documents or managing a to-do list.

Third-party stuff: Just months after Apple opened up the iPhone to other developers, thousands of programs are available, and downloading them directly via the App Store is a cakewalk. The best ones, such as Facebook and the Evernote note-taker, are outstanding. But the limitations that Apple puts on third-party apps--they can't run in the background or access data other than their own--place major obstacles in the way of everything from instant messengers to office suites. And Apple, the sole distributor of iPhone software, has declined to make available some useful applications that developers have submitted.

Bottom line: iPhone OS is easily the most enjoyable and intuitive phone operating system in existence, but its growth could be stunted unless Apple keeps its control-freak tendencies in check.

Next: Google Android and RIM BlackBerry OS

Google Android

What it is: Google's new phone OS is an ambitious open-source platform intended to let companies customize it to their liking for an array of handsets. So far, however, it's available on just one model, T-Mobile's G1.

How it works: On the G1, Android's interface feels like an iPhone/BlackBerry mashup--much of it uses the touch screen, but you get a trackball and Menu, Home, and Back buttons, too. The highly customizable desktop is a plus. Overall, it compares well to older platforms but isn't as effortless as the iPhone.

How it looks: Android isn't an aesthetic masterpiece like iPhone OS, but it's fresh and appealing, and it makes good use of the G1's high-resolution screen.

Built-in applications: They're tightly integrated with Google services such as Gmail and Google Calendar--the first thing you do when you turn on the phone for the first time is to give it your Google account info. (That's fine as long as you're not dependent on alternatives such as Microsoft Exchange.)

Android's browser lacks the iPhone's multitouch navigation but is otherwise a close rival. The best thing about its music features is the ability to download DRM-free songs from Amazon. The only videos it can play are YouTube clips, alas.