I would consider buying a warranty for a mission critical device provided it gave me an immediate replacement.
Otherwise, I break out my trusty American Express card which includes a built in warranty protection solution. It double the manufactureres warranty up to 5 years (excluding cars.)
It has never let me down. I simply call them to place a claim or do it via their website. I give them the purchase date info, receipts if needed, and depending on the pheasabillity they either repair or replace the unit.
Most recently I ran over my mp3 player. Instead of fixing it they gave me the $180 to get another one.