Personally, I would only buy a warranty, if I feel the device in question has a tendency to fail.
So far, I have only bought one warranty and it was worth it. A Dell laptop came with 12 month warranty, but after 14 month, the video card broke. Dell replaced the motherboard+video card and at the same time offered me to extend the warranty by 2years for 120£, even though the warranty had already expired 2 month earlier. Since then has the laptop been repaired two more times, with the Display and DVD drive being replaced. I am not entirely sure, how much all of this would have cost me without warranty, but certainly more then I paid, not to mention that the Dell repair service came to mu house for free to repair the items.
As a result, I would always observe how the device performs throughout the first year and then only at the end of the year decide, whether warranty is a good idea.