Which Newtons have you used? I've used every one of them, from the first (when they got cheap) to the last, the MP2100, with a 160Mhz StrongARM CPU.

The Newton was, and still is, the most innovative PDA on the market. The UI is far superior to the Palm, with many widgets that have been carefully optimized for pen-based usage. Most PDAs now use widgets that look remarkably like those on desktops - ignoring the fact that they are designed for mice, not pens.

The first Newton's handwriting recognition was not that good, unfortunately. However, the Newton OS was about a lot more than that. The apps are tightly integrated with each other. If you want to send a document by e-mail, or by any other means (FTP, IRC, infrared, and any other protocol somebody implements a transport for) you do it from within the app, rather than having to go, find it in your file system, and tell your e-mail system to add it as an enclosure.

Global preferences are implemented very nicely - there is an extensible framework allowing applications to change their settings based on the work site you are at. I can specify that, at home, I use a particular modem or wifi AP, a specific printer, and that I want my e-mails to be sent out immediately. On the road, I can specify that my outgoing mail is stored in my outbox till I get online, etc. etc.

When you use the built-in Notes program to create a document, it formats it for your screen. It can automagically reflow it to fit an A4 or US Letter page when you want to print it. It can actually print from any app directly to a printer - something that doesn't seem to be possible from most PDAs out there now.

Applications can be easily installed on your built-in storage device, or on a card, and moved back and forth. They can even be sent to other people! This is something that I haven't actually seen any other PDA do.

The Newton uses a persistent object store rather than a file system. This means that storing your documents doesn't require first selecting a name and storage location. You can add metadata later for filing purposes, but you don't need to do it before your data is saved.

"Find" is global - you can look for particular information in _all_ applications, get a summary list, and then go to that particular data within the app that created it. If you can't remember where you wrote about a particular topic, this is extremely convenient. It's very similar to Spotlight on OS X.

Really, though, you have to play with one to really understand. Apple elegantly solved problems that most people didn't realize existed. If they'd kept up with the Newton, I think it would easily blow away everything else out there now.