The UI for managing apps on the BlackBerry is pathetic. There are at least four places that apps can reside on the device, so finding them is an unwelcome Easter egg hunt. (You can move some of them around to consolidate the mess.) On an iPhone, they're easily and consistently accessible, and infinitely easier to organize than on the BlackBerry. You can download "themes" for the BlackBerry that change how apps are organized, including some that unify them into a common location -- these themes are third-party add-ons, not something the BlackBerry provides itself. But the BlackBerry does let you create your own folders, so you can manage your apps however you want; the iPhone only lets you rearrange your apps, not organized them in folders.
Most BlackBerry "native" apps I tried were just glorified WAP apps, not real apps that take advantage of device-specific capabilities, as native iPhone apps do. (WAP is the DOS-like mobile "Web" technology that the cellular industry tried to palm off on us in the late 1990s.) BlackBerry apps -- at least so far -- are incapable of doing the cool things that iPhone apps can do, whether acting as a level or a credit card terminal, managing your Amazon.com orders, or translating foreign-language terms (even hearing the pronunciation, which was handy on a recent trip to Portugal). Awkward interfaces make many BlackBerry apps painful to use, and they usually cost two or three times as much as their iPhone equivalents.
The iPhone has a real OS, and its SDK lets you create real applications, with menus, buttons, interactivity, video, forms, and so on. Plus, you can use Web apps, getting the iPhone's UI for HTML-based functions such as fields and pop-up menus; you can even save the Web apps alongside your other apps for quick one-click access. By contrast, the BlackBerry apps often consist of browser forms and buttons (often at tiny, unreadable sizes) that fetch and display data from the Web. RIM might like to think of them as native apps, but they're really just stubs to Web apps.
Most apps available for business are either personal aids such as tip calculators and expense logs; front ends to sales tools; or basic editors. The iPhone has better UIs for the first two types of apps. For editing, the BlackBerry has DataViz's $70 Documents to Go (a basic version is included at no charge by many carriers), which is capable and straightforward, letting me do basic text edits in Word, Excel, and PowerPoint documents, and simple formatting such as boldfacing text. You can cut and paste as well. Tracked changes are removed from the document, and though extensive editing is theoretically possible, you're hamstrung by the device's keyboard and trackball.
On the iPhone, I used the $20 Quickoffice for iPhone, a productivity editor that has similar capabilities (including internal cut and paste), plus retains any revisions tracking in the original document. But it can't work with zipped files. Quickoffice is a little easier to use than Documents to Go, but Apple's prohibition against saving files on the iPhone means that Quickoffice can't get to those e-mail attachments. Quickoffice does have a cool tool to transfer files to and from the iPhone over Wi-Fi, but you need your computer up and running to do that -- in which case, why would you edit the documents on the iPhone? Recently released, an iPhone version of Documents to Go can download attached files if they come from an Exchange Server, which only partially gets around the Apple limitation; but it works only on Word files, so it's not terribly useful.
I also tried the devices on Google Docs. It's barely possible to edit a spreadsheet in Google Docs on an iPhone; the most you can do is select and add rows and edit individual cells' contents. You can't edit a text document, and for calendars all you can do is view and delete appointments. The BlackBerry lets you see spreadsheets one column at a time -- which is useless. Bottom line: You won't use Google Docs on either device.
I found several BlackBerry apps to be unreliable and very slow. Salesforce.com, for example, didn't open for weeks due to an undefined error when connecting to its site. When I finally got it installed, it was very hard to read and use. I tried five times to download Gokivo Navigator -- BlackBerry App World's top-rated navigation app -- at half an hour a pop. It worked the sixth time, and 90 minutes later was installed and running. Not only did the installation take nearly 45 minutes, but then it rebooted the BlackBerry, which took another 45 minutes to grapple with whatever changes were made. This simply doesn't happen with iPhone apps.
When all was said and done, Gokivo Navigator turned out to be hard to use compared to the iPhone's Google Maps. It has as many confirmation dialog boxes as Windows Vista -- so getting to a result requires many clicks -- but lacks the real-time scrolling or page-by-page direction features of Google Maps. You'd need to be desperately lost to use it -- and forget about accessing it in a moving vehicle, given how slow it is and how hard it is to mouse through the maps. The alternative is to pay a monthly fee for AT&T's voice-based navigation service, which is available on many phones, not just BlackBerrys.
I also found that several BlackBerry apps often hogged my device's resources, leaving me unable to switch to another application, the Web, e-mail, or the phone. That can happen on an iPhone as well, but the "stuck" times on the BlackBerry were both much more frequent and longer in duration. The BlackBerry's application switching issues meant that its alleged advantage of being able to run multiple apps simultaneously is limited, essentially letting you pick up where you left off rather than really working with multiple apps. Still, that's more than the iPhone can do.
I did find one BlackBerry app advantage: the ability to open files in zipped attachments (a glaring omission from the iPhone).
If you want to use apps on a mobile device, the BlackBerry is not a realistic option. If your work forces you to use a BlackBerry, get an iPod Touch for the apps.