I've got a Palm Treo 650 and it's pretty handy. Being able to access my contacts and calendar is invaluable, and being able to surf the web and play backgammon is nice.
Today, the device became a lot more useful for my job. Google release Google Maps for Treo which give you real time access to maps and driving directions and traffic and aerial photos. If you have a Palm (and a good data package with your phone provider), you need this. Check it out.