It also depends on how you use the laptop and how much you depend on it. If you are using it for 15+ hours each day and cannot afford to be without it for any prolonged time, then a good warranty is a must! I've been very satisfied with Dell's next day on-site service: a tech hired by Dell would just come to my office and do the repair/replacement on the spot quickly and professionally. $100-150 for en extra year of such service (for a $2500 laptop), I think is a good deal. In my personal experience the investment in the extended warranty usually pays off. But even when it doesn't, it gives me peace of mind!
And as mentioned above, I too wouldn't buy the extended warranty for a netbook or an entry-level laptop. It's better to save the money and just buy a new one once it breaks.