I think that the Palm-style devices are the biggest PDA development of the '90s. Apparently, this is what the masses really wanted: a good organizer that is small enough to be carried around, but big enough that it doesn't hamper working on it too badly.
Just before Apple announced the Newton's demise, I started looking for other PDAs to develop for. The Palm was very popular at the time, and I thought I'd take a look at it. I bought one, and started developing for it immediately. My reactions were mixed: the Palm's core functionality (date and address books) is incredibly well engineered. People that buy Palms buy them for this single function, and the Palm's success is explained by the general good behavior of these applets.
On the other side of the coin, I found it very difficult to develop for the Palm. Its API is still immature, and the development tools that were available at the time were either wildly expensive or total junk. I ported my favorite starter project (SameGame) to Palm and then stopped developing for it. The development environment never matured enough for me to return.