I have to agree with Chris as to the usability of iPhone vs. Windows Mobile. I recently
got an iPhone for my wife and it's awesome. It's just so much easier and smoother to use. Now as to whether you can really use it for business, I guess that depends on what your business needs are. I found the Outlook sync to be primitive- it only syncs contacts in your "Contacts" folder (i.e. you can't pick which Contacts folders you wish to sync, it only syncs the default one), and same for Calendar. And if you need some specific 3rd party app, then of course your choice is obvious: the iPhone isn't for you. But 3rd party app support is supposedly coming next year, so the business use scenario for the iPhone may improve.
Example of ease of use of iPhone: Monday we were moving in to a new house and we decided we wanted to order pizza for ourselves and the movers. We have no phone book and don't know any pizza places around here. So I whip out the iPhone, turn it on, go to Maps, pull up our new home address from a bookmark, type "pizza near " before the address and press "Go" (or whatever the button is labeled). And a few seconds later it shows a map with a bunch of pizza restaurants near our house. I find the closest one that isn't a Pizza Hut and select it. It shows like a contact- with name, address, phone #, web page, etc. I tap the phone # and it dials up the pizza place and I order my pizza. How easy is that? I know I could do the same thing from my HTC Advantage or any one of my many Windows Mobile devices using either the Google Maps application or through PIE or other browser, but it would have taken way more screen presses, typing, stylus use, etc. and the results wouldn't have been presented nearly so nicely and it wouldn't have been as easy to dial up the resulting phone #. The iPhone is quite simply a joy to use (in most scenarios). Using my Windows Mobile devices now feels clumsy and awkward, when just a couple weeks ago I was pretty happy with them (in blissful ignorance to the fact that there was something better out there).
My 3rd party app use is mostly limited to apps that fix the sorry built-in apps that Microsoft provides (Like CorePlayer for WMP). But the iPhone's built-in apps suit most of my needs as they are, so I might not need 3rd party apps like I do on my WM devices.
And Chris is also right about Word docs looking way better on the iPhone than in Word Mobile on Windows Mobile. True, you can't edit them without web apps (yet), but they render very similar to how they render in desktop Microsoft Office Word. So it's a great way to check out document attachments to email.