It depends on what the product consists of
You might consider buying the extended warranty if:
a) If the product has mechanically moving parts (e.g., hard/CD/DVD drives), and or, high voltage parts and battery packs it might be worth it, check the fine print to see if they are covered.
b) the unit has a very short factory warranty (less than 6 mo. or so)
c) you intend to operate the unit 24/7, or under environmentally stressful situations (hot, humid or dusty environment)
You might consider not buying the extended warranty if:
a) the unit has no mechanically moving parts, no high voltage parts and no battery packs. It can be statistcally shown shown that if such units survive the first month of operation, they will last for a long time, way beyond the expiration of a 1 or 2 year extended warranty if the units are operated in a normally controlled home or office environment.
It is precisely this fact that resellers count on. In other words the most reliable operating period for electronic low voltage devices is between roughly 100 hrs of operation and design life, which might be 10,000 hrs if operated within the specified environmental limits. If operated 5 hours a day, this translates into about 5.5 years after which time the reliability will decrease. Thus, you would be buying warranty for the most reliable period of operation for the unit.
Another consideration is, of course, the cost of the warranty. If the cost is more than say a hard drive or a battery pack for a lap top, then it's not worth it. If you consider the odds of a failure being 50/50 over the covered period, then the cost of the warranty should be one half of the cost of the most vulnerable part of that unit.