I've found that most professionals don't like being "available" during off work hours. The exception are workaholics and those who are self-employed.
Some even have two cell phones.
So, if your work has you on the road, or frequently in a situation where access to a computer and the internet is limited - a phone/pda will probably increase your productivity. However, if you almost always have access to email while working, you'll probably rarely use the pda features, and if you don't use it often enough - you'll probably discover that after the initial usage, you'll never use it again.
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.