What advantages does WinCE have over other handheld palmtop operating systems such as the Newton and others?
Wow! Now that's a leading question. Here's the short version of the answer:
First, I presume you've seen the news that Apple has dropped the Newton, so not much point in considering that one.
While some other handheld devices (like Sharp's "Wizard" product line) offer many of the features that are available on CE-based devices, none of them offers the same breadth of options. More importantly, the general lack of support for development makes it difficult to create custom applications on those other platforms. So the biggest advantage is the ease with which developers can create applications targeting Windows CE handhelds. This translates into ready availability of applications at a reasonable price.
While you asked specifically about handhelds, a word about palm PCs is in order. US Robotics' Palm Pilot is a great device with a huge following. Because developers have good tools (such as CodeWarrior) and information for that platform, it will undoubtedly remain a viable product for the foreseeable future.