To the OP, I'd be curious to know what your lifestyle is. I.E. Are you a student? Run a business? Work for a company? Work in a certain field? I think that plays a major role in helping you discern whether or not iPad is a good fit for you.
Me, for example; I am a college student and aspiring filmmaker. I have a MacbookPro and an iPhone 3GS, both of which I take with me on a daily basis to class, Starbucks, library, wherever I go to do my work. What sold me on the iPad was a few features;
1) Textbooks. You might not read books, but I have trouble believing you don't occasionally want to read a magazine, newspaper, or even a novel. In my case, if publishers eventually offer textbooks on the iPad, it will be incredibly convenient rather than lugging around five or six books in a backpack. We've already seen some videos of publishers showing off apps and magazines for the iPad, and the videos are pretty durn amazing.
2) Expandability. Nobody knows what amazing apps are going to come for the iPad, but with such a large screen, I bet some great stuff is coming. I could totally see developers creating a handwriting recognition app that uses an accessory stylus, which could be great for someone like me who takes notes in class (maybe even a Livescribe-type app without the fancy pen; that would be SWEET!).
3) iWork Suite. Maybe you don't use iWork, but chances are you at some point have used Office. Having a suite of applications on an iPad that are DESIGNED for a multi-touch interface is pretty awesome. I'm looking forward to not having to lug my MacbookPro with me everywhere. When I want to head to Starbucks to write an essay, do a presentation, or review my notes, I have it on a .5-inch device that will last me around 10 hours. And, per point 1 above, if I have my books with me on that device, referencing them should be simple (I know multi-tasking would be great for this, and I'll hold out hope for that, but even if that never comes, I'm still sold on it).
4) Screen. Lastly for me, the screen size is a huge seller. When I want to show off my latest video, I can export it for iPad via Final Cut Pro, save it to my iTunes, pop it on my iPad and show it to someone. No need to open software, wait for a program to load, play, go full screen. I open the Videos app, I tap the video, I'm done. That's going to be so awesome.
5) Accessories. I'm not entirely sure on this one yet, based on the fact that I can't try it, but I love to travel. I take photos with my digital camera, upload them to my wordpress blog, and write stories of my experiences. If I can hook my digital camera right into my iPad, upload the photos right from there to my wordpress blog via a wordpress app, and post, I've just eliminated the need for my notebook. I love my MacbookPro, but sometimes it's much easier to have a thin tablet-like device with me, rather than my laptop and bag.
I don't know what I would call the iPad (a tablet? a computer? a notebook?), but I'm sticking with its something in a category of its own. I had trouble decided whether I need another $499 device, but in the end, it actually offers some great features. If you can't come up with a single reason you need it, maybe you don't. Not everyone needs everything. But if you give us a bit more about what you do and what you use your notebook/desktop for, I bet we can help you decide whether this is a worthwhile investment.
P.S. I'm totally holding out hope that someone is going to develop a small device that connects to my wifi network and is nothing more than an IR transmitter. Couple that with an iPad app that allows me to customize a remote interface, and I've got myself a web-browsing, e-mail reading, VNC-capable tablet universal remote control. Since the Logitech Harmony tablet-esque one is $399, I'll happily drop the other $200 for the extra features
