I'm a happy user of Windows Mobile 6. I've had some not-so-good phones in the past, although my latest - the HTC Touch - really is a nice piece of kit. My commitment with WM is because I really need my Outlook details on me at all times. (I used to use a PDA for this before I carried a mobile with me routinely.) I've tried other phones that claim that they can sync with outlook, but it never works.
Even though the iPhone is very tempting, I haven't succumb because I can't easily sync all my Outlook information to it. Somewhat amazingly, Nick pointed me to this interview with Terry Myerson [link]. It turns out that Microsoft has just done a deal with Apple to enable them to sync with exchange and embed active sync on the iPhone!!! Wow - I didn't see this coming! Once an iPhone appears with this feature I'll be first in line :-)