Any time I buy something that involves a warranty, I use these criteria to decide if the warranty is worth the money:
1) Will the item still be useful and "up to date" by the time an extended warranty could be needed? Or will I want to replace the whole unit by then any way?
If the item is likely to be replaced before an extended warranty comes in to play, then the extended warranty is pointless.
2) What would it cost to repair the most likely failures in the device, and is that cost more than the warranty itself costs?
If a single repair costs more than the extended warranty, and I intend to keep the item for a while, the warranty is probably worthwhile.
3) Is the item something that my kids are more likely to break? If so, does the extended warranty include accidental damage/breakage?
For something like a plasma TV that can be easily cracked by errant flying toys, an accidental damage warranty is probably a good idea in my house where small children (i.e. walking miniature tornadoes) roam.
4) Does the warranty require that the item be mailed in to a central repair facility at my expense?
If so and it's a heavy item that will cost a lot (and take a long time) to ship, I probably won't want to use the warranty even if I need it, so paying for the extended warranty is a waste of money.