Apple Inc's iPad hit stores on Saturday after months of buildup, igniting excitement and kicking off a critical sales period that will determine if the sleek tablet computer becomes the next blockbuster.

At Apple's flagship store on Fifth Avenue in New York City, cheers went up from employees as shoppers entered the shop at 9 a.m. (1300 GMT), emerging a few minutes later carrying the devices touted as a bridge between a laptop and smartphone.

As Wall Street gauges the device's popular appeal, an infectious energy permeated the crowds at many stores.

Tim Bajarin, president of consulting company Creative Strategies and a long time Apple watcher, said lines at the stores were long and enthusiastic.

The device lives up to its hype, this really has the potential to reset people's expectations for how you use a computer, he said.

Buyers whooped and pumped fists as Apple employees applauded and high-fived the first customers.

Hours after the doors opened, the store remained loud and bustling. Some customers, like Simon Cox, immediately cracked open their new toys for a trial run.

Cox, a math teacher visiting from Manchester, England, said he sent a quick email to friends and family.

Noting that the iPad was smaller than he expected, he said: It's easier to carry around. I certainly know I'll use it when I'm out and about.

Technology experts also rushed to have a firsthand look, taking apart the gadget to examine its components and workings. One firm, iFixit, an Apple parts and repair specialist, revealed the iPad includes chips from Samsung Electronics, Broadcom, and Texas Instruments.

Wall Street wants to see if the device, which went on sale at the company's more than 200 U.S. retail outlets, along with many Best Buy stores, can win a mass following.

If so, it could provide another boost to Apple, whose stock has been hitting record highs, as well as companies that provide parts and components.

Crowds built steadily, with shoppers waiting at stores in

New York, Washington, Boston and San Francisco. But the lines were noticeably shorter than those that ushered in the iPhone in 2007.

Will Kiefer, 28, who waited at Burlington Mall, north of Boston, said he hoped the iPad would allow him to break away from his desktop computer and avoid buying a laptop.

I think this will do everything for me that I need, said Kiefer, a freelance software developer who hopes to develop some apps for the new device.

In Richmond, Virginia, about 100 people gathered at an Apple store.

Matt Reidy, IT director at a company called snagajob.com, said he got there at 1 a.m. and was first in line. My wife thinks I'm crazy, said Reidy, 43. She said I'd be the oldest person out there.

Because customers have been able to pre-order the gadget since mid-March, there was little reason to stand in line. Those who ordered early enough online were to get their iPads on Saturday, from store pickups or home delivery.

Analysts say the company received several hundred thousand pre-orders, with sales estimated at between 4 and 7 million in the gadget's first year.

MUCH AT STAKE

Apple has plenty riding on the iPad, which it introduced in January and calls a new category of machine: a lightweight media consumption device that tries to fuse the best attributes of a smartphone and a laptop.

The iPad's touchscreen measures 9.7 inches. At 1.5 pounds (680 g), it resembles an oversized iPhone and runs on the same operating system. It starts at $499 for a short-range Wi-Fi model, topping out at more than $800 for a 3G-enabled version.

The iPad is designed for using media of all sorts, including games, video, pictures, e-books and magazines. It can access roughly 150,000 existing iPhone apps, as well as new ones freshly designed for the iPad.

Apple is also launching its own digital book business to compete with the Kindle from Amazon.com Inc and other e-readers and e-books.

The iPad is the first in a wave of lightweight tablet devices expected to land this year from rival vendors, including Hewlett-Packard Co and Dell Inc.

The question is whether the iPad can attract a mainstream following beyond the first few months of excitement.

Technology enthusiasts have praised the iPad's beautiful screen and fast Web browser, but also have pointed out some missing pieces. It lacks a camera, cannot run more than one app at a time, and it cannot view popular video sites that use Adobe's Flash software.

Reviewers at The New York Times and The Wall Street Journal said the iPad works nicely for Web surfing and multimedia -- but may appeal less to people who need computers for more heavy-duty chores.

Saturday's iPad launch is only in the United States, and only for the Wi-Fi model.

(Additional reporting by Gabriel Madway in Richmond, Va. and Ros Krasny in Boston, and Christopher Michaud in New York; Editing by Alan Elsner)

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