An article on OSViews has a great history of the development of hte Newton MessagePad: Think you know the Apple Newton's History?.
I owned a MessagePad 2100 and it was an amazing little machine. I did a lot of business travel at the time, and it was a great road warrior--I could check email, send faxes, write documents, keep my calendar, addressbook, etc. It even made a great alarm clock. I later tried a Palm III but it could not live up to the Newton. Not until the iPaq, many years later, did I feel the industry had finally caught up to the Newton, and then only when you added Calligrapher (the original Newton handwriting recognition software).