I guess it depends on what the extension costs, and if you've had any problems in the past that you're worried about. Keep in mind that an extended warranty is a bet. You're betting the motor will explode, Merc is betting it won't. Make sure the warranty is through Mercury and not some independant warranty company. I've heard horror stories about getting a claim paid. If you have had good service from your motor in the past, and take care of your stuff, the money spent on the extension could be better spent elsewhere. Just my opinion, I usually back down from extended warranties. I'm sure some people swear by them.
fog