Let's start with a caveat: I haven't seen or handled this device yet. Indeed many hours after its launch it wasn't even on Apple Australia's website, though it is now. Rather I'm basing my response on Apple's published material.
Firstly, this thing will sell, and sell lots. Apple is a good marketing company and this will sell. I've no doubt that it will change and morph over time and that will make it sell more. But right now here's my scoreboard.
iPad = iPhone - voice + screen size - portability
The key issue here is that this thing feels like a first edition. We don't see any startling new developments in multi-touch; we don't see any startling new capability; it seems like a larger, less portable iPhone. I think the screen real estate will provide an awesome browsing, reading game-playing experience. The problem is that iPhone "works" because it's in your pocket. Not because it's the best and brightest screen to work on or the simplest interface, rather it works because it is the best compromise and above all it's with you.
Not so the iPad, it's too big to be "always-with-you" and I'm sorry Steve but I don't think it's small enough (or sexy enough) to be "intimate".
For me though the killer is that it isn't a productivity machine. If this was to work for me I need to be able to leave a gadget at home when I travel. At the moment I travel with iPhone and MacBook. I need iPhone for voice communication and I very much like its app ecosystem (though not its closed nature) and its pocketability. I use the MacBook for real work though, composing lengthy emails, working on documents, spreadsheets, databases. In addition my MacBook has two Java apps that I cannot do without. One of them is an XML editor and the other is the local client for our Component Content Management System. I can't travel without them. Simple, end of story.
In addition every bit of productivity work I do sees me switching between applications - often the email app and the browser with a word processor interspersed from time to time. Or alternatively the local client and the XML editor. The iPad doesn't support multi-tasking and even though I'm a bloke and I'm not supposed to be able to, I do multi-task.
That means that when I travel I still need my iPhone - the iPad has no voice comms capability (yes I know about VOIP over 3G but I need real ubiquitous calling capability); I also need my MacBook because the iPad won't run my workday applications. The question then becomes "Does the iPad add sufficient value that I can add it's 600g or so to my carry on baggage?" Despite the welcome addition of iWork the answer is no.
There is one "maybe" though. It does make me wonder whether you might ditch the iPhone and revert to a $100 simple mobile phone and then the iPad might have a place alongside the MacBook. In the end I think that it just means that your bag got heavier for not enough reason really.
So the iPad doesn't create a place for itself in my bag. Three simple additions would get it there though: multi-tasking, support for Java apps and a move away from the closed App Store ecosystem to allow me to place my applications on there. Oh and a fourth thing: a real and accessible file system.
Beyond that I'd like support for modern wide-screen formats, HDMI out, an iSight style camera (making a cool Skype conferencing device) and a true "next-gen" multitouch interface.
What do you think?