512KB may not seem like a whole lot of memory, but you'll be amazed as to how much information that translates into.
If you have problems trying to input data while you're in a hurry, then do as I do, I carry a small writing tablet that I use for quick notes, and then I enter them into the REX.
I had an Everex unit (similar to the Palm), but it was too bulky for me to carry around comfortably.
The tiny Rex gets my vote. I love it!