Extended Warranties -- maybe
I tend to not buy them. All of the previous comments about researching the product ahead of time are important... I always consider what the cost of repairing or replacing the item is versus the cost of the warranty... and obsolescence of any given device is also important.
Generally, if I'd be more likely to buy a new device than repair it I won't even consider the warranty. If it's something with a long lifespan or that has some feature I can't do without, I'll consider the warranty. And yes, manufacturer warranties are always best.
Perhaps most importantly, figure out how to be a "pack rat" with all of those receipts. It's one of the reasons I use my Visa card for all of these sorts of purchases, as there's an additional level of tracking there.