The Newton, while a very, very good machine for its time, has been slowly losing ground ever since it was discontinued. It's still a great machine, and it still has advantages over many competitors of _today_, but it's getting more and more of a drag to synchronize with either a Mac or a PC, as incompatibilities are growing. It's still very good at what it does, but newer needs can only be fulfilled with workarounds.
A Newton MessagePad 2100 (the only machine I'd still buy today) is a great PDA, even more so if you can find the right software for it.
But at the end of the day you might just be better off with a newer device from Sony or Palm.
For example, the Palm m515 is a very sleek little machine that has a great display which works well under all lighting conditions. So does the Sony NR-70 or the 615c/625c (depending on where you live, the devices are named differently at Sony).