Through the iPad, which fits somewhere between phone and computer, Apple must convince people who already have smartphones, laptops, ebook readers, set-top boxes and home broadband connections that they need another device that serves many of the same purposes.

Many of the earliest iPad buyers say they will have a better idea of what they will use it for only after they have had it for a while.

Beth Goza, who has had iPhones and other smartphones, along with a MacBook Air laptop, believed the iPad has a place in her digital line-up, likening it to a professional tennis player owning different shoes for grass, clay and concrete courts.

"At the end of the day, you can get by with one or the other," she said outside an Apple store in University Village mall in Seattle, Washington.

Apple says its tablet-style iPad computer represents a new category of consumer-electronic devices, ideal for watching videos, surfing the web and reading electronic books. Here is how it compares with other internet-connected portable devices consumers are already using:

• iPad vs laptop
The iPad is easier to pack and carry than most laptops. The device weighs just (680 grams), compared with a few kilograms for the typical laptop. At a mere 1.25cm thick, the iPad is also thinner than most laptops. But with a maximum of 64 gigabytes of storage, the iPad cannot hold as many photos, films and songs. The iPad comes with only a touch-screen keyboard; Apple will sell an add-on dock with a physical keyboard like most laptops have.

• iPad vs netbook
Smaller laptops known as netbooks use less-powerful chips than regular laptops, and as a result they do not handle video or other processing-intensive tasks well. The iPad uses a new chip custom-designed by Apple and which chief executive officer Steve Jobs has described as extremely fast.

• iPad vs smart phones
Browsing the web, watching video and reading books are more comfortable on a big iPad than a tiny smart phone. The iPad's screen measures 24.6cm diagonally, nearly three times that of Apple's iPhone, 8.9cm. However, while the iPad has a built-in microphone and could work with internet-based phone services such as Skype, it is not a telephone. And it does not fit in your pocket, the way the iPhone does.

• iPad vs e-readers
Using the iPad's touch screen to buy books and start reading seems fast compared with the navigation required on Amazon.com's Kindle, which requires pushing physical buttons on the device because it has no touch screen. But the iPad has a glossy screen, so it might not be as easy on the eyes as the Kindle and other e-readers, which generally sport greyscale "electronic-ink" screens that are not susceptible to glare.

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