Apple's iPad is just one year old, and more than 15 million customers have voted with their wallets. The tablet is officially mainstream.

But just what is a tablet good for? It's not a complete replacement of a PC and it's not a necessity for anyone who carries around a notebook or a smartphone. At best, it's a "tweener" device.

For a product category that didn't exist (except in niche form) a year ago, it's surprising how well the tablet is doing. It's as if a mainstream product appeared out of nowhere. Indeed, most tech experts underestimated how many iPads would sell in year one.

If it's not a necessity, doesn't do many things as well as a notebook and lacks the portability of a smartphone, what's the key to its success?

Perhaps the best gadget to compare with the iPad is the microwave oven, says tech writer Matthew Guay. Succeeding the conventional oven, the microwave oven could heat food faster and use less energy. Even though it wasn't as good at cooking as an oven, and it wasn't obvious why anyone would want a microwave, the microwave became a staple in practically every home, because people kept finding new ways to use this technological wonder.

It seems like the same thing is happening with tablets.

"Everyone thought the iPad needed traditional computer programs to be successful. After all, if you can't use Office, what’s it good for?" Guay wrote.

And then customers bought them, took them home, and something special happened. They realized that reading eBooks or browsing the internet from their couch was nice on a tablet. They found things they would have never thought to do on a computer were fun and simple. Apps that never made sense on computers with keyboards and mice, like GarageBand and finger paint apps and eReaders, suddenly found life on a 9.7-inch slate of glass and metal.

Indeed, it turns out that a tablet needn't do everything that a more powerful PC can, according to multiple research studies on iPad usage. Rather, the tablet's main appeal lies in the approachable touchscreen interface that just about anybody at any age can pick up and figure out.

As you might expect, the top three things consumers have been doing with iPads are surfing the web, writing and checking e-mail and playing games, according to a study published last year by NPD Group. iPad owners are also watching video and reading e-books, and the device's light weight and portability make it a real crowd-pleaser, NPD found.

"While lots of choices and compromises go into the development of any product, especially something as different as the iPad, these results indicate that most consumers are satisfied with their purchase and are increasingly finding ways to interact with their iPad," NPD wrote.

Additionally, a casual poll conducted by Gadget Lab on Twitter asked the question "What do you do with your iPad?" and the majority of respondents said they used the tablet for browsing the web, reading (books and/or news articles) and social networking.

The minority of respondents to Gadget Lab's poll said they used the iPad for special purposes such as recording music, writing poetry and teaching in class from book notes.

"Read, use it to teach from (presentation notes in iBooks) and email," said iPad owner Josh Smith, in a Twitter reply to Gadget Lab. "Occasionally write up posts in bed w/ silent keyboard."

As for apps, the most frequently downloaded apps are in the Games, Entertainment and Utilities categories. However, TruVoipBuzz looked closely at the numbers and found that those top three categories only account for 46 percent of apps that attracted the most downloads.

The rest of the pie is divided into smaller slices: People are downloading apps from a wide variety of categories, including photography (4 percent), books (4 percent), social networking (7 percent), business (3 percent) and others. (See the chart below.)

That's the genius of the blank slate – with nearly 400,000 apps that allow the iPad to become a toy, a TV, a medical tool for doctors, a notetaker for students and more, it caters to an extremely broad audience.

The truth about the iPad is that there is no dominant group of "joe schmos," creative customers or professional customers buying it. And if you look at it that way, the fact that 15 million iPads sold in one year isn't completely surprising, after all.

If you have an iPad, how do you use it? Let us know by voting in the widget below. If you use it for something we haven't listed, you can submit additional uses (they won't appear until they've been moderated, however).

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