Eager customers intent on being among the first owners of Apple's iPad, some from as far away as Europe, queued across the US to buy the newly-released tablet computer.

They seemed willing to buy first - and discover uses for the iPad later, in a scenario in some ways reminiscent of the hoopla surrounding the 2007 launch of the first iPhone.

But the difference is that people knew then that the iPhone would replace their existing mobile phone and the appliance has become a must-have for everyone from uber-geeks to stay-at-home mums.

With 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.

She said she was already dreaming up specific uses for her iPad, such as knitting applications to help her keep track of her place in a complicated pattern.

Danita Shneidman, in her 60s, wanted one to look at photos and videos of her first grandchild, born this week in Boston; and Ray Majewski, who went to an Apple store in Freehold, New Jersey, with his 10-year-old daughter, Julia, said the iPad was partly a reward for her straight-A school grades and partly a present for himself.

The iPad is essentially a much larger version of Apple's popular iPhone, without the calling capabilities. Just a half-inch thick, the device has a touchscreen that measures 9.7 inches on the diagonal - nearly three times the iPhone's. Also, like the iPhone, it has no physical keyboard.

For now, Apple is selling iPads that only connect to the internet using wi-fi. Those models start at £329. Versions that also have a cellular data connection will be available by the end of the month and will cost £85 more, with the most expensive at £545.

In Apple stores in Seattle and on New York's Fifth Avenue, the atmosphere was festive, with employees cheering and clapping as customers entered and left.

But once the initial iPad excitement settles, Apple may have to work harder to persuade a broader swathe of people to buy one.

Many companies have tried to sell tablet computers before, but none has caught on with mainstream consumers. And while early adopters who pre-ordered an iPad in recent weeks have gushed about all the ways they hope to use it, sceptics point to all the ways the iPad comes up short.

They argue the on-screen keyboard is hard to use and complain that it lacks a camera and ports for media storage cards and USB devices such as printers.

They also bemoan the fact that the iPad cannot play Flash video, which means many websites with embedded video clips will look broken to web surfers using Apple's Safari browser.

And the iPad cannot run more than one program at a time, which even fans hope will change one day soon.

Lower prices could encourage more buyers, but when Apple slashed prices for the iPhone just months after its release, early purchasers were irate.