Apple may sell as many as 200,000 iPhones in the product's first two days on the market after sales begin at 6 p.m. Friday and as many as 3 million in the second half of the year, according to the most optimistic analyst estimates. Apple, in its only public forecast, says it plans to sell 10 million next year.

Sales at those levels would outdo the iPod, Apple's best-selling product to date, for comparable periods. The danger is that Apple may fall short of projections for initial sales and dampen investor enthusiasm for the product.

''There's definitely a lot of buzz,'' said Andy Hargreaves, an analyst at Pacific Crest Securities in Portland, Ore. ''If they sell only 100,000, that would be bad.'' Scrutiny of the product is so great that any small disappointment could send Apple's stock plunging, experts say.

Skeptics wonder whether even the most innovative product could live up to the iPhone's lofty expectations - and whether the pre-launch anticipation has spiraled too far out of control.

Technology analyst Mike McGuire said Apple fans have elevated the status of the iPhone to unprecedented proportions - ''somewhere between electricity and sliced bread.''

''The blessing is you've created an amazing amount of demand. The curse is you have a very high level of expectations to meet,'' said McGuire, a research vice president with Gartner Inc.

Apple claims the iPhone will be easier to use than other smart phones because of its distinctive touch screen display and intuitive software that allows for such user-friendly features as scrolling visually through voice mail messages and easy access to the Internet and video and music libraries.

Apple Inc.'s two most lucrative products are a study in contrasts. Its iPod dominates the digital-music device market with more than 70 percent of players sold in the United States, while after decades the Macintosh has secured barely 5 percent of the personal computer market.

So which one is the iPhone destined to emulate?

That depends on whether early buyers like it enough to match the hype, whether Apple lowers the price soon and whether the company can quickly learn the ropes of the complicated cell-phone industry.

''When people first have it in their hands, they will be thinking more with their hearts than their heads,'' said Michael Gartenberg, research director at Jupiter Research. ''But quickly, everything about the iPhone has to meet consumer expectations.''

The stakes are high for Apple, which is counting on the iPhone to join the iPod and Mac as its third major business. So is Wall Street. Apple's shares have soared more than 40 percent since the day in January when the company announced the product, which combines a cell phone, music player and Web-browsing computer into a touch-screen device.

The importance of Apple's first-ever phone is magnified by slowing growth of iPod sales. Analysts say that music-playing cell phones, often subsidized by mobile carriers, pose the biggest threat to the iPod's dominance. The iPhone will probably accelerate that trend, as some consumers shopping for a new iPod will buy the iPhone instead.

The iPhone has already generated a thriving cottage industry online, with more than 1,100 peripheral iPhone items for sale on eBay, including colorful holsters, touch-screen protectors and car adapters.

But the hype has also hurt Apple.

The launch is being so closely watched that Apple's share price plunged more than 4 percent in a matter of minutes last month after a rumor about a delay was reported on Engadget.com, an electronics Web site.

The rumor was quickly corrected by Apple, and the stock largely recovered by the end of the day. Apple's stock closed on Monday at $122.34, off 66 cents.