Having been a user iPod's for years and recently changed to a Mac from a PC at home I guess I am a fairly zealous proponent of Apple, but trust me the iPad is more than just a bigger iPod Touch.
I got my iPad to supplement my work laptop, now I love gadgets but hate carrying my laptop around with a passion. What I mean is I used to carry it back and forwards to meetings and all I did was show one presentation or take a few notes, and I still have my iPod Touch with me to keep me entertained at airports, on planes and trains and when in hotels. The iPad means I can now travel in most cases without my laptop, and indeed have now made quite a few business trips without my laptop and have not been disadvantaged at all. Travelling with just my iPad has been distinctly liberating as it just lightens the load when deciding what to take away. Ok, the reality is that this freedom has meant spending a few £ on apps in the iTunes store, but it's worth it.
So onto the functions... Calendar, the built in calendar is great and easy to use, and combine this with your Mac calendar or Outlook and you have a very powerful tool. Music, the iPod functionality is great and very easy to use, the controls are pretty intuitive. Photo's, being a bit of a sentimentalist I like to take family pictures with me when I go away, and the app on the iPad works very well and is a great way to view pictures and the picture frame mode is good too. Video, viewing films and tv programmes on the iPad is a great experience. The screen is a god size for viewing and the picture quality is superb. Boring stays in hotels are now much more enjoyable!
There are many other great built in features, but I guess you've read about these elsewhere in one of the myriad of reviews available. And as for 3rd party apps, there is a huge choice, the main issue is finding the ones that you 'really' need and that do what you want, but there are some great ones out there.
In terms of other accessories I needed to make the iPad a success, the main one was a MiFi, this is a great bit of kit on the 3 network that creates your own private wifi hotspots wherever you are as long as you can get a 3G or better signal. I went for a pay as you go option and have been very pleased with it. I also use an AirPort Express when abroad or in hotels to create a wifi link from the cabled Internet available in meeting rooms and hotel rooms when no wifi available. (data roaming abroad is expensive so I try to avoid the MiFi when abroad!) Then there was the VGA connector to allow me to connect unto a projector to show presentations, combine this with Keynote, Goodreader of 2Screens and you have a good presentation tool.
Now all I am waiting for is the iOS4.2 release so I can use AirPlay to connect to my AirPort Express and stream music when staying in hotels. This update may also entice me to get an Apple iTV but that's another story!
All in all I was very impressed with my iPad from day one, and my opinion has not waned over time.