As someone who carries a iPhone 3G ver 3.0 Beta phone, Windows Mobile 7 Beta phone (Touch HD), and a Symbian Series 60 v5 phone, I can venture a somewhat informed opinion here : the iPhone wins, but just. Its all down to whether you want a capacitive or resistive touch screen. My preference is capacitive, because I find it faster and easier to use FOR ME, and I couldnt be arsed using a stylus. Exchange 2007 integration is also another biggy : the iPhone is just so much better at synching Mail, Contacts and Calendars with Exchange servers.
The devil is in the details : with the iPhone you can query Active Directory for company wide telephone numbers straight from the contacts page, so you don't have to pollute you Contacts list with work people (provided of course your company maintain the GAL properly). The iPhone is also far more usable when it comes to watching media or listening to music while on the run, so if you travel a lot, its a no brainer. I also find the iPhone handles handover between 802.11 and WCDMA a lot better than the others, without all the stupid prompting and confirming that comes with Win Mobile 7 and Symbian.
I am eagerly awaiting delivery of my Omnia HD... 720p home movies from the phone. Sheez.... The possibilities are endless