As I just mentioned in this thread, I have an iPAQ hx4700 with Resco Photo Viewer software.
I didn't buy the iPAQ for photography - I use my Pocket PC for many things, not least because I can't write now due to my disabilities. However, it has an excellent 4 inch 640 x 480 screen, and built in SD and CompactFlash type 2 slots, so it makes a good review device. However, if your main need is for a device for photography, I'd buy an Epson P-2000 or similar - something with a built in hard disk to download your memory cards.
I've no experience of Palm OS, but there's some hints that it's reaching the end of the road as it stands, and that there will be a jump to a future, possibly incompatible, (Linux based?) OS. My impression is that Microsoft Windows Mobile for Pocket PC is probably the best of the high end PDA OSes at the moment, though it's a pain to use a Pocket PC if you don't have a desktop or laptop running Windows. There are ways to use a Pocket PC with a Mac, but there's no official support, and you'll probably have to find a friend with a Windows machine to install some commercial software.
The choice of which PDA OS to go with is like choosing Canon versus Nikon. I nearly went for Palm OS when I switched from the Nokia Communicator devices around three years ago, but am so glad I opted for Pocket PC.
David