A number of years ago I had and used several PDAs, mostly Palm PDAs but also at least one iPaq Pocket PC PDA and other similar devices. After a long hiatus I am now a daily user of a Palm PDA again. A Palm m500 model is my choice because I think it represents the best combination of features that this era of PDA had to offer.
These are the reasons I think a Palm PDA is still a highly useful device still today, some 20+ years past it's heyday.
Some of these points are specific to the m500 and similar monochrome PDAs. All of these things add up to a device that melds some of the best aspects of digital and analog. Being digital, it can store as much personal information, notes and books as you would ever need without growing in physical size. An analog notepad or planner has an advantage of never needing to charge and always being ready to do its job at an instant no matter how long it has been sitting idle. The m500 is nearly as convenient with its low power nature and having such good battery life that you hardly ever need to think about charging it. In my experience the 10 or so minutes that it sits in the cradle each morning to sync my gemini digest is enough charge to keep it going perpetually.
It can do a whole slew of genuinely useful things that we typically use our smartphones for with an order of magnitude better battery life, outdoor usability and without opening yourself up to the distractions and security/privacy implications of always being online. It is my address book, calendar, notepad, project organizer, ebook reader, grocery list, distraction free word processor, doodle pad, todo list, calculator, dictionary and TV remote. Not to mention there are plenty of games and I read gemini on it.