I use a Palm PDA (Vx) and find it excellent, especially when used in conjunction with a desktop PC to update/backup data.
Very convenient to keep appointments, schedules, etc. Wish that the company would send the schedules in some computer compatible format, though :-)
Email is possible with a GSM phone but veeery slow until UMTS or at least GPRS come along. It takes ages to download mail so this is only if you really need to get some mail while on the run.
I use PalmLog Pro to track flight times mostly to be able to get totals and check for currency/legality. At last I'm able to answer the "hours flown since last medical" question........
I use it in parallel with a paper logbook which seems more "permanent" and "personal" to me than the printouts I could possibly create with the palm. If currency calculation, etc. is not needed I wouldn't go for an electronic logbook, filling the paper one is easier........
I hardly ever leave home without the PDA and it is much more useful than the Sharp "Wizard" I had a few years ago.
As for keyboard/no keyboard, that's the main choice to make. The "grafitti" handwriting required for all Palm-type machines is easy to learn though you keep correcting the occasional letter if you start to write quickly. No keyboard allows for miniature size AND a good screen, though.
Color is nice but not really required as the b/w units have the whole software optimised for the screen. Color units are battery hogs and eat up batteries fairly quickly.
I can recommend the Palm but you have to try if the "grafitti" feels "right" for you.