Last weekend I showed my friend my iPhone (1st Gen iPhone) and let him use it for a while. He had just picked up a Windows mobile phone a couple days before and he sat and compared the two phones. His phone was the Verizon Wireless XV6900 I believe.
Here are a couple direct quotes made by him.
"I think the iPhone has a better user interface then my Windows Mobile phone"
"The windows mobile interface is just terrible"
My experience is the opposite. I've had the iPhone for two weeks and ran WM5 on a Treo 700w and 700wx for two years.
Worse stability by far on iPhone - I've had more crashes, program quits and freezes in two weeks than I did in two years with WM5
It takes a lot more swipes touches and key presses to do simple things
WM is definitely easier and faster to use one-handed
WM it's easy to
Keypad (subjective and I'm not fully used to it yet) was quicker on a Treo, not requiring constant button pushes to flip between keyboards.
lack of office support stinks
attaching docs to email
no cut and paste
ability to easily edit the registry
FTP
Bluetooth was better
can use phone as storage device
sd cards for big files
IMHO, Wm is better for serious business users. iPhone is nicer and simpler for many tasks and media use. 3rd party progs for WM blow away what's available for iPhone, but this will likely change as time goes on. WM's biggest problem in my experience was active sync, but even that allows much more than mobileme.
While not high-tech a dpad is extremely useful and in many (not all) respects faster.