I've been a Windows Mobile user since the old days of the Compaq iPaq, the HP Jornada, and the monochrome screen devices. In fact, they were called Windows CE at the time. Then they were called "Pocket PC."
I also had the original Palm Pilot, but I quickly abandoned that device to switch to Windows Mobile so that I can multi-task and keep several applications open simultaneously.
I'm currently a Treo 700wx user and I'm running Windows Mobile 5.0. Palm hasn't released a Windows Mobile 6 update and I don't think they will for this device.
However, there are many improvements in Windows Mobile 6.1 that really attract me. Specifically, the mobile Internet Explorer will have zooming features and "full page view" called a "Page Overview." Adobe Flash will be available on WM 6.1
These features will make WM 6.1 devices almost comparable to the iPhone's Safari web browser. In my opinion, the iPhone is much easier to use to browse sites, but I need a hardware keyboard. Maybe the next version of the iPhone will have a hardware keyboard that slides out.