The iphone was revolutionary because it packed a lot of functionality into a device that i needed. I need a phone. the fact that i can have a phone that does other stuff and plays cool games makes it a killer platform. It successfully combines my phone and my DS into one unit that i will always have.
I also need a computer. the macbook does a wonderful job there. it's portable, it's powerful, i can make stuff on it.
The ipad is just this thing that sits there and you would only pick it up occasionally. i'm sure it's a nice e-reader, though i can't wait for kindle for osx. i don't have problems reading on my macbook. It would be neat if it had a comprehensive tv remote and my tv somehow worked over wi-fi. my iphone could do that too though.
To decide where these devices fit in my life, i imagine packing for a trip. I would take my phone. I need my phone, and hey, i can play some games on it and check emails, etc. I would probably take my macbook. i still like to putz around in blender and unity on vacation. the ipad obviously isn't going to work well for content creation. at this point, i have no reason to bring the ipad. the ipad seems to have a permanent spot sitting next to my couch. That seems like a stupid thing to spend $500 on.
On the other hand, i think in the future we will have tons of these devices. Your microwave door will be a transparent display that is all one big touch surface. it will display info about what's inside via little labels that appear to hover over your food. Your car's instrument panel will probably be the same. Something like the ipad will cost much less. You will take it for granted. You won't think twice about packing it because you can always get another if it breaks, etc. You won't feel like you wasted a bunch of money on a thing that just sits by your couch or bed because that's all you will expect of it.
I think those future devices had to start somewhere. technically, i think they started on phones, but this is another step towards that future. Magical and revolutionary? Probably not. Another step towards appliance computing? maybe.
I'm not running out to get one now. In 5 years why wouldn't i spend $50 on one?