In my opinion cell and smart phones are a very personal devices and many people are passionate about what they like. I personally have used quite a few phones and almost all models of blackberrys. We have had to support a number of WM devices and I have never cared much for them (personal opinion). When compared to the effort to support a blackberry on BES WM tends to be more effort.
While iPhones are easy to manage on Exchange they are not without their issues. Email and contacts seems bullet proof but Calendar syncing has issues. Especially on reoccuring meetings that have been updated. It is constantly screwing up my calendar with respect to timezones and reoccuring meetings.
In the end you can argue about what is out selling who or what is better; however it comes down to what you use it for and what you value more. It is hard to get an apples to apples comparison.