A single use (once per device) OS... ... The feelgood speeches cannot deny that wholly. Fremantle will fix current issues in Diablo, require new hardware, allow the community to inherit some of it's architecture, while basically becoming a deadend itself shortly thereafter.
So is it worth the upgrade? If you desire any of these functionalities, Windows Mobile 2003 SE is definitely worth a look, but just because you want it doesn't mean you'll be able to get it...