Preamble: I'm a Software Designer and Programmer (aka Software Engineer)
I have at my disposal, 1x iMac 24", 1x MBP 13", 1x Mac Mini, 1x iPhone, 1x iPad
I use ever single piece of this equipment for work. I use most of it for home. I use some of it for play.
The iMac is a must for personal data management, talking Aperture, iTunes, Finances, etc. I also use it for working from home, as well as managing backups and such things. It's the mule of the household in computing terms.
The MBP is my primary workstation for work. I can do 100% of my job with that computer alone if necessary. I avoid using it for personal purposes as much as is possible to avoid any kind of inadvertent slip of NSFW content or software license issues.
The Mac Mini is used for managing archives at work, for running regression testing, and repeat small scale performance testing (so I can keep on working while tests are running without slowing them down)
The iPhone makes calls
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Though rarely, I hate talking on the phone. I do most of my e-Mail, IM, SMS, Skype, etc. on this device. At least, I used to. The iPad has changed my iPhone usage a bit, more on that momentarily. One thing that has grown and continues to grow is how much I game on my iPhone. It has become my #1 gaming system, despite owning all current gen game systems and multiple computers. It's just too compelling to pull out the ol' iPhone which is always on my side than go find or set up one of the old consoles.
And finally, the iPad. I explained the uses of the other items so you'd understand how the iPad is fitting into an already seemingly crowded toolkit.
I use the iPad for work: reading PDF, reading books on safaribooksonline.com, technical documents, drawing diagrams (especially in ad-hoc design meetings), taking notes, managing calendars, contacts, and now it is my preferred device for correspondence mail (email that doesn't require me sending off any deliverables.)
I use the iPad for home: *COUCH COMPUTING* I hate that modern laptops light your lap on fire. Also they are heavy and floppy. I can curl up with the iPad in my lap, browse casually, never turn it off and have battery life all night long. I spend a lot more time around the family since I'm not drawn to the iMac just for catching up on my various RSS feeds and flickr account. I watch podcasts, tv shows, listen to music (especially when I'm doing other things on it) and some gaming. I really like Labyrinth 2 HD and am in love with Radioflare Redux HD. Warpgate HD, and flight control. I never got into flight control on iPhone but the large play field makes me enjoy it a lot more.
I use the iPad for hobbies: I am an amateur photographer. I like to use the iPad to view my photos after a long shoot to get an idea of what I have and what I'm going to be doing in Aperture while I unwind from a long hike or whatever. I also love the iPad for showing off any pictures I'm proud of. I also use it for recipes. Pull up a recipe, drop the iPad in a large ziploc bag and get to cooking. I read eBooks exclusively, have for years. I really like the large screen when it's split like iBooks does. It's a shame the Kindle app can't do this, as I find the screen is too large otherwise.
All this, and I've only owned the iPad for a week. In the first week that I owned my iPhone 3G, I barely did anything besides twitter and record my spending. Almost two years later I have a huge number of apps (mainly games) and use cases for the iPhone with more on the way with iPhone OS 4. I have no doubts the the iPad will continue to mature and developers will start to realize the potential over time. The effort to produce a computing experience that is defined by software as much as possible is one I can really get behind. The least enjoyable aspect of computing, to me, is all the unnatural human-machine interfacing.
Now it's true I could do almost all of these things with my other devices but in most cases, I could not do it as comfortably, for as long, and with as little baggage. The iPad is what I would call a companion. It's not required like my phone is, and it's not as complete a computing device as the MBP, but it sticks by my side for more conveniently. It's barely heavier than carrying a professional notebook around and does so, so much more.