The Pre, Droid, and Droid Eris have limited security capabilities (the Pre has the most and the Droid Eris the least) and no management tools. Companies that offer mobile management tools are increasingly adding iPhone and Android support, but the scope of these tools remains less than regulation-concerned enterprises require.
The BlackBerrys, iPhone, Pre, and Motorola Droid all support VPN connections; only the HTC Droid Eris doesn't. The BlackBerry's support is superior, as its VPN settings can be remotedly managed by IT. Using the free iPhone Configuration Utility, IT can also send iPhone users VPN profiles to install, but can't remotely manage them.
Although what distinguishes smartphones from cell phones is data access, smartphones still must function as phones and handle voice calls. Except for the iPhone, all have good voice and speaker quality. The iPhone stands apart: Even rabid fans complain about its poor voice quality and frequent dropped calls. It's tempting to blame AT&T for that, except that the BlackBerrys don't suffer these issues on AT&T's network.
All the devices integrate the phone with their address books and search functions. But the Droids' search capabilities are more primitive than those of the other devices, You can't search just your contacts, for example.
The iPhone set the bar for mobile usability upon its debut and has continued to raise it with the two major OS releases since. The gestures make sense, and the OS and apps are full of little enhancements that bolster ease-of-use, such as the quick-delete swipe and ability to bulk-delete and bulk-move messages, as well as the ability to add Web pages to the home screen's apps grid for easy access. More recently, the iPhone added very intuitive, "smart" copy and paste that works for text and graphics. Where the iPhone falls short is its lack of support for running multiple apps simultaneously, which means switching apps can essentially close an app rather than freeze its current state for later resumption; this one-app-a-time limitation prevents the iPhone from being able to open file attachments and edit them from e-mail, for example. As previously noted, the Pre is the only device that makes it easy to move among multiple apps simultaneously -- as if you're using Mac OS X or Windows. The Droids' ability to pop up a menu of active apps (by holding the Home button) is not as good as the Pre's approach but still better than the iPhone's.
The HTC Droid Eris is surprisingly easy to use and full of well-designed applications that come in very handy. HTC has created its own Sense UI that extends the Google Android UI. While there are a lot of naked rip-offs from the iPhone, the UI is no mere collection of robotically stolen features. As mentioned earlier, the many widgets bring capabilities together intelligently, with many pleasant surprises in terms of options and display. The Motorola Droid has none of these, using the raw Android UI instead. It's a decent OS, but rougher by comparison to what HTC has done. But do note that the HTC Drid Eris has a slower processor than the Motorola Droid, so it's not as snappy switching among or running complex apps.
The Palm Pre's UI is clunkier than either the iPhone's or the Droids'. It shares many of the same principles, but is less consistent in, for example, its use of scrolling and menu access. Also, the separate swipe area for some gestures is confusing, as it's hard to remember which swipes work where on the split screen.
The BlackBerry UI is vintage, and not in a good way. Very reminiscent of Windows 3.0, it uses clunky menus, keyboard shortcuts, and primitive imagery. The trackball is jumpy and inaccurate, so scrolling, selecting, and zooming are imprecise activities. Keyboard aficionados like its extensive use of shortcuts, but there's no reason the BlackBerry OS couldn't retain those while introducing a modern graphical UI.
Beyond UI, the iPhone has also introduced some cool capabilities that let it and its apps do more. For example, an accelerometer allows player-style games and even tools such as carpenter's levels. Also, the built-in compass lets map-based applications become direction assistants. The Palm Pre and the two Droids have copied both capabilities, and the BlackBerry Storm has an accelerometer.