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Most consumers who have shopped for portable electronics, home appliances, or automobiles know that after settling on a product and price, the buying process is rarely complete.

Retailers of such goods commonly pitch warranties for repair service down the road.

Also known as service agreements or product guarantees, extended warranties generally cost between $30 and $500, and last from two to five years. They offer added protection and peace of mind for consumers who worry that standard manufacturer warranties, which normally last between 90 days and a year, will lapse just before their machine malfunctions.

Yet arguments against product guarantees are compelling.

The costs can be artificially high. Retailers generally reap substantial profits from these agreements. Consumers Union, a Yonkers, N.Y., watch-dog group estimates that retailers' profit margins on extended warranties range from 40 to 77 percent.

On top of that, consumers rarely benefit from the added expense. Only 20 percent of those who buy electronic devices with extended warranties ever use the service, according to the Consumer Electronics Association (CEA).

In fact, most problems with consumer electronics are found when they are taken out of the box, or within the first 90 days of use, says Sean Wargo, a senior industry analyst with CEA.

"The vast majority of returns happen within the first month of ownership," he says.

Many items, like VCRs and DVD players, are often so cheap that they can be more easily replaced than repaired. Boston resident Chris Capano simply bought a new compact-disc player when his old one broke.

"It was a cheap CD player, and it wasn't worth the hassle to buy the extra insurance," says Mr. Capano.

In the case of computers, some experts recommend pitching the product altogether - recycling it, ideally - to take advantage of new technology usually included in the latest models. A recent dip in prices on both desktops and laptops makes this option even more attractive.

Those seeking added protection, however, aren't limited to purchasing extended coverage. Consumers can double the length of the manufacturer's warranty by making purchases with certain credit cards. Providers include Visa's Gold, Platinum, and Signature cards.

A number of home-insurance policies, according to Mr. Wargo, also cover a variety of problems relating to owner mishap.

The decision on whether to buy a warranty largely hinges on the particular product involved. Paul Roberts, president of Newtonville Camera in Newton, Mass., says customers should consider choosing a service agreement for expensive devices that require specialized knowledge to repair.

Mr. Roberts sells additional warranties on items like video recorders that cost $2,000 or more. A five-year warranty would likely run between $200 and $400, he says, while repair costs often total $300 or more.

Dan Martin, owner of Martin's Appliance & TV in Struthers, Ohio, normally discourages customers from buying extended warranties. But he does make some exceptions. Expensive items are often worth protecting, he adds, particularly if they might wear from frequent use. And a product's length of time on the market should be taken into account.

"Big-screen or high-definition TVs are so new on the market, they might have more bugs," he says. And if you buy "a high-end washer and dryer for a family with eight kids, you should get the insurance."

Still, Martin believes customers should spend more on better products rather than buy insurance for cheaper models. He sells a $100 extended warranty for a $300 Frigidaire washer, but he suggests customers buy a $400 Whirlpool and not worry about repair problems.

The time consumers must wait to get back their repaired device grows each year, according to Martin, because of an increasing scarcity of parts, even for newer models.

And, in Martin's experience, the service firms that are contractually tied to service-warranty holders are not always very reliable.

"There's not a 'Johnny-on-the-spot' [repairman] that runs and gets you taken care of," says Martin. "The aggravation isn't worth the money."

As for automobiles, warranties are generally considered a wise investment. Some dealers will even refund warranty payments if the owner never requires service. Buyers usually have about a year to decide on a policy. If you plan to keep a new car beyond the duration of the manufacturer's warranty (usually three years or 36,000 miles) consider buying the extension late in that first year, to avoid too much overlap.

Experts point out that auto insurers and credit unions also sell car warranties. The Kelly Blue Book identifies one website (www.warrantybynet.com) as a reliable source for a quote on an extended warranty for an automobile.

Shoppers who decide to get an extended warranty on any item should be aware that the terms of service agreements vary among retailers. Stores will often negotiate the price to encourage a sale, but it's important to remember that most service agreements begin at the date of purchase and overlap with the manufacturer's policy.

For example, CompUSA's warranty on a $1,500 desktop computer from Compaq requires a $420 payment for three years of in-home service. For the same model, Circuit City offers four years of service for $300. Home visits cost extra. Three-years of in-home service from Best Buy costs $200.

But manufacturers themselves often pitch much better deals. Compaq offers an additional two years of at-home service on top of its one-year warranty on the desktop model for only $150.

The final option: Fix it yourself. Undoubtedly, few people are comfortable picking apart complex circuit boards. Yet some very basic tweaks can add a year or two of life to a major appliance.

Chris Hall, president of RepairClinic.com, says most people are unnecessarily daunted by basic repair. Mr. Hall, a former appliance-repair technician, says appliances are consistently updated with new technology, but that they are also getting easier to fix.

"Most of the parts have become modular assemblies, and can be popped out with a couple of screws," says Hall.

Consumers buy parts from his website for complex jobs, says Hall. Common purchases include water pumps, electric switches, and small motors.

Still, he says that for those with little to no repair skill, extended warranties are a reasonable option.

"If they're prone to worry about an appliance breakdown, and feel comfortable having an insurance policy to protect them, I say go ahead and get them," says Hall.