Ok I'm on my iPhone right now!
I've had various PDAs over the years - palm pilot, compaq one, loads basically, also had a Sony ericsson P910 then a P1i. The iPhone beats them all hands down.
I sync mine via ITunes with Microsoft outlook on our home laptop, so does DH, so I can see what we're all doing. DH also syncs his at work - he has a separate work calendar I think, but I can look at that too if I want to (quite useful, you know how crap men are at remembering appointments etc)
I also sync all my contacts with outlook, and my notes.
I can pick up all my ntlworld emails easily (also easy to set up) as well as work emails.
Web access is fantastic and user interface is excellent. MN access is great, as is online ordering from eg John Lewis sites.
I didn't think I'd be bothered about iTunes but it's great to be able to plug it into an iTrip in the car to listen to music, including some of the kids' music. Saves having CDs rattling around the car!
What else? The only downside is I've developed a sudoku addiction . There are some useful applications like currency/weight etc conversions, notes, weather forecasts, loads! The google map feature is great, essentially a sat nav (a basic one)
Oh, it's a good phone too.
I couldn't be without it, and I'd never go back to a separate mobile and PDA. I think the 8gb iPhone costs around £95, with a monthly charge of £35 for about 600 minutes and 400 texts