AH! Something I can comment on...
I love my Vx- though if I could justify the expense I'd get the M500. Alot of the new software is HUGE, especially if you want to do silly things like play video clips or show pics of your dogs. The 500 is essentially the Vx with a faster processor and has the memory expansion slot. Nice. Color is unnecesary for most people, I think. The rechargeable battery is very nice, as you should be hot-syncing to back it up every night anyway. It's just a fire-and-forget operation: stick it in the cradle, press the button, go take a leak, come back it's done.
The best feature of the Vx/M500 series is the size. All the others are pretty big, and I really like having the little booger in my front shirt pocket, ready at a moment's notice. It tracks appts, the evergrowing list of phone numbers and addresses, events into the next two millenium (avoid those "Did you remember our anniversary is next tuesday, dear?" situations), lists, lists, more lists, Quicken checkbook, and so on.
I usually run:
ePocrates and Medscape's Pharmacopeia (OK, a little redundant...) www.medscape.com
Datebk4 (MUCH improved Date Book) http://www.pimlicosoftware.com/
Backup Buddy (essential!) and WordSmith (MUCH improved Word-compatible doc reader and To do/Memo substitute in one) http://bluenomad.com/
Oh, and my current fun game: Bubblet www.oopdreams.com
There are currently (and going to be many more) apps for the medical field, including wireless order and prescription functions, E&M coding in the palm of your hand, billing programs, etc. Many medical professionals have them, which makes networking easier ("Beam me your business card, man!"), letting you do a better job of minding your business, IMO.
Just too useful, not to mention too cool not to have one with you at all times.
Good luck on your decision!