They CAN be worth it...but usually aren't
I usually avoid the extended warranty. Most electronics with few or no moving parts don't break down...and if there will be problems, they typically show up almost immediately.
That said, I was once "talked into" purchasing the extended warranty for a DVD player. The rational was that the warranty was cheaper than a single visit to the repair store and that the laser in the DVD players typically goes out of alignment within the extended warranty period. I bought it...and within a year-and-a-half used it. The extended warranty (in that case) definitely paid for itself.