To see how today's smart phone OSs stack up, I spent time with five leading ones as experienced on phones that show them to good advantage: Apple's iPhone OS (which I tried on the iPhone 3G, using AT&T's network), Google's Android (on T-Mobile's G1), Microsoft's Windows Mobile (on HTC's Touch Diamond, using Sprint), Nokia's S60 3rd Edition on Symbian (on the company's N96, sold only in unlocked form), and RIM's BlackBerry OS (on the company's own BlackBerry Bold, using AT&T). (Consult PC World's Top 10 Smart Phones chart to see how the hardware compares.)

I judged the five operating systems on their capabilities, ease of use, and visual panache, and considered both their standard applications and third-party programs.

The Winners

The two most impressive operating systems were the two with the briefest histories: iPhone OS and Android.

Both are built for Internet-centric devices, both are not only functional but fun, and both make extending your phone's capabilities with new applications extremely easy. At the moment, iPhone OS beats the newer, rougher Google OS; over time, Android's open-source design and lack of restrictions on third-party developers could give it an edge over Apple's more locked-down approach.

Among the old-timers, the BlackBerry OS is doing a solid job of preserving the strengths that made it popular in the first place while keeping up with the times. In contrast, I regret to report, Windows Mobile and S60 3rd Edition are aging badly.

Let's delve more deeply.

Apple iPhone OS

What it is: iPhone OS is a pocket-size version of the Mac's OS X, shrunk down and redesigned to power the iPhone 3G.

How it works: As you zip your way around the iPhone 3G's multitouch interface with your fingertips, hardware and software blur into one pleasing experience. With other OSs, it's all too easy to get lost in menus or forget how to accomplish simple tasks; iPhone apps, however, are remarkably sleek and consistent. The OS's most infamous omission is cut-and-paste capability--but to tell the truth, I haven't missed it yet.