The iPad (or other tablet devices) is not a pure laptop replacement. In many ways, it is much more and much better. In some ways, however, it is inadequate for the job. The iPad isn't so much a computer, it's more an all-purpose media / creative potential interaction portal (though more and more vertical industries and industry in general are finding ways to use it as an efficient and/or transformative enterprise tool).
There are definitely some things (e.g. heavy word processing) for which you'd find a tablet to be a poor substitute for a laptop. On the other hand, a tablet is superior (far, in my experience) to a laptop when it comes to many of the things that a lot of people primarily use their laptops for. The internet browsing experience is much better with a tablet in my opinion. Reading content (i.e. magazines, books) is easier and better. Playing games, watching movies, listening to music - all appreciably better and fuller experiences with a tablet. Tablets also open up creative potential in ways that laptops simply cannot - creating music (e.g. with garage band) and visual art is possible now, even for the novice - it's right there at your fingertips.
It's hard to explain the enormous real potential of these devices - as the devices themselves are, sensing that potential is experiential. They are much more interactive and convenient devices. I can't recall opening my laptop since I got my iPad (I may have once or twice, but at this point I don't remember doing so). Then again, I never used it for some of the work type things others surely use their laptops for.
Whether or not a tablet would work as a laptop replacement depends heavily on the needs and uses of the person in question. I definitely would not generically recommend an iPad (or other tablet device) to someone as a laptop replacement. If you do a lot of word processing or have some other specific work related needs for a laptop, a tablet may be a horrible fit for you. For me, and I'm sure many others, a desktop (or two) for when I (they) need to do certain kinds of things and an iPad/tablet (or two
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) for everything else, and for mobility and convenience, fill out my (their) needs and wants. A laptop is a completely unneeded and redundant device.