I upgraded my iPaq hx2415 from 2003SE to WM5 and now i own an x51v that came with it preinstalled.
After using WM5 for a while I cant imagine ever going back to 2003. I gave the iPaq to the wife and juts last night she was playing a game and it locked up the device. Soft reset had no effect. I told her to remove the battery and of course she screamed at me for the suggestion. So I yanked the device from her shaking hand and removed the battery and reinserted it. The iPaq booted up and everything was intact except the date and time. After that I gave the iPaq back to wife and offered to reload WM 2003 on it. She in turn offered me the couch to sleep on.
Sorry for the long story. Here are my reasons for loving WM5...
+ Persistent storage means no more data loss
+ Improved formatting for Office documents and support for PowerPoint
+ Dedicated RAM & ROM (No longer dynamic. IMHO this is a plus)
+ Soft Keys (individual opinions will vary on this one)
+ Updated Internet Explorer (Now view full screen and save images etc)
+ Support for Photo contacts in Pocket Outlook
Here is a page with more info:
http://pocketpccentral.net/wm5brief.htm