A lot of people take exception to my "WinCE is great" comments. They are all right -- the UI sucked. I'm mostly referring to the internals which were well-suited to the ARM CPU capabilities at the time.
Early mobile CPUs had no MMUs, or limited/weird MMU features. Thus, they couldn't have "real" operating systems like Linux or Windows, but some compromised operating system.
The MMU is the feature that separates kernel memory from memory used in apps. It's a stability feature, meaning the system doesn't crash when the app does. It's a security feature, meaning hostile apps can't take over the system.
WinCE started in the days with no MMU's or these weird limited MMU techniques ("slots"). That was its problem: as a first mover in the smartphone market, it was tied to the past.
So WinCE's operating system internals did a good job at being an operating system for the technology of the time, while still presenting the programmer with a well-known API they could program to.
But at the same time, the UI was a joke. You were trying to shoehorn concepts made for a desktop GUI into a device that had at most 240 pixels wide. It's like a 2 year old playing the piano: judged not against adults, but that they can do it at all.
