Financially it's not worth it unless you buy some relatively rare, expensive, laptop like an ultraportable 1.5lb machine that nobody else has within a 100 mile radius. Otherwise, any lemon from a mainstream system will show itself within the first 30 days usually, majority will just be DOA. All companies offer at least a 1 year free warranty period (if not, RUN like hell) so issues like a messed up keyboard will show itself up. Aside from that, if you buy a mainstream laptop such as Dell, HP/Compaq, then there are so many common parts you can buy off the shelf from ebay in the event of a problem, such as broken hinges, etc.
Remember that warranties only entitle you to have an exact spec'd machine returned to you. Does not neccessarily have be the machine you originally owned. So your pristine no dead pixel UXGA screen machine may in fact be replaced with an identical UXGA but with dead pixels all over (as companies usually have ridiculous dead pixel acceptance standards). Frankly, this issue in itself turns me off from buying any extended warranty as a laptop with a perfectly flawless screen can be a difficult process.
With laptop prices going down and continuing to fall, there is even less reason to buy an extended warranty. Remember that companies push ext warranties hard because they are huge profit drivers.