REPLACEMENT plans are typically fine on high-use stuff (game-consoles, phones, etc..) as its very simple to convince BB or CC that the item no longer works like "brand new". Say the phone has static, the controllers don't work, etc...they have to replace the whole thing with a gift-cert of the original price.
So buy a console now, 2 years from now get that money back to buy a new console.
Service stuff, I'd say no except on stuff that pays for itself (sometimes they include another battery on camcorders), big TVs as well...
But replacement is typically fine I'd say. And I've worked at BB and CC.