I think you're right about not being "available", although I do like to be "knowledgeable" and aware of what's going on even if I don't reply. I think I've found that during work hours, when I'm away from email, (which is roughly 15% to 20% of a 40 hr work week), I'd really like the access.
However, when it's not "work hours", I don't see myself using it.
On the other hand, I rarely if ever use my PDA for personal calendar, tasking, etc., but I think I would if it were a part of my phone.
Also, I find myself "neglecting" my PDA more often than not (letting the battery die, not cradling it to sync it up with the desktop, etc.) because I pay more attention to the cell phone. I'm thinking if it were a part of my cell phone, I'd pay more attention to it.
Maybe not though. I'm still weighing it all.
Chances are, you're probably the type who does better with an actual date/address book.
My father has tried the PDA thing for years and he always falls back on his date book. He just has a secretary update it every couple of days by hand.
One use for the PDA/phone that you might find useful is to set up a new email address for use with the smart phone. Once that is done, you can set up an email filter on your work computer (if you leave it on), and messages that meet certain criteria will be forwarded to your smart phone's email. That should cut down on the emails you get on your smart phone.
The question is do you actually keep your computer date books and address books up to date? If not, don't expect to start just because you bought a smart phone.