Check the start date, too. Some multiple-year service plans start from the day of purchase, even if you are covered by the manufacturer's warranty for a period of time. For fewer administrative hassles, you may also want to deal with a service that doesn't require you to pay service bills and then seek reimbursement from the warranty provider. Not everything is included in the plan. Computer coverage often extends to connected peripherals in addition to the mouse and keyboard.
Service levels can also vary.
As a bonus, some services even offer replacement equipment if it needs to repaired for the same problem multiple times. Extended service plans are not cheap, though. Manufacturers typically charge considerably more than third party providers. Service plan pricing is often highly negotiable, however. If you want an extended service plan for all your equipment, which can cost several hundred dollars, it's worth shopping around.