A lot depends on the warranty plan you purchase. A simple extended warranty doesn't go as far as the the Geek Squad Protection plan and purchasing the accidental damage plan.
Years ago, we got my son a laptop from Best Buy and we did the full Geek Squad thing. They actually did pretty well. The hard drive failed at one point. He got the thing fixed and a full system restore, as I recall.
Anyway, it worked out for us. You really have to pay close attention to what each plan covers.
I do think it's sloppy, though, to have the cashiers push these plans. When you're waiting in line and have people waiting, you're less likely to make a good decision. You'll either quickly turn it down, just to save time; or you'll buy it, without really thinking about it. There really isn't much time in those situations to talk in detail with someone about different options.
It's really best, when buying a computer, to get a salesman and go over each plan carefully to really understand what you want. That way, when you come to the register you can make a more informed decision.