That wasn't always the case, and not that long ago. Years ago I remember the never-ending quest to find the most portable device of the time that could still handle all that I needed to do on a regular basis. Laptops were big things to lug around regularly, so I constantly experimented with using other types of devices to get stuff done.
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I was not alone in that quest, I remember corresponding with a novelist in the UK who hated dragging the laptop around just in case he needed to write. He tried all sorts of gadgets that failed in one way or another, and eventually settled on using his Windows Mobile PDA with a portable folding keyboard.
His criteria were simple -- he needed to have the tools with him all the time. It needed to have good battery life to last all day and he wanted to just pull it out and get working with no fuss.
While that method wouldn't be good for most folks, this guy spent hours all over the world writing several novels with the phone/ keyboard combination. His reasoning was simple -- he never knew when a writing opportunity would present itself, and he needed to be sure he had the tools at hand when it did. The PDA and folding keyboard fit in his shirt pocket, so that's what he used.
The mobile worker of today has much better choices, and that complicates finding the right method. Some folks need a full laptop all the time, and the highly portable netbook fills that need. The new Ultrabooks coming to market are a good fit for those folks, too.
Others, and I am in that camp, find tablets to be a good tool for mobile work. They are thin and light enough to be highly portable, so they can meet the "at hand" need when work opportunities present themselves. Some tablet users find the onscreen keyboard to be all they need for working, so the tablet alone is just right.
Tablets are a personal thing, and fortunately there are all sizes available to fit the individual. The 5-inch Galaxy Note may be all some folks need, but for those wanting a bigger screen there are plenty to choose from. The key is to try several first-hand and see what works best.
Some of us pair a portable keyboard with the tablet, and that's just fine. It comes down to personal preference, and what makes it easier to get work done wherever (and whenever) the opportunity presents itself. As that novelist told me years ago, the best tool for the job was the one he has with him at the time. All other criteria fall by the wayside if the tools are back at home.
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