it depends on what store you're looking at and what product you're buying. the quality of the warranties varies from store to store, and how much money we make on them varies as well. i work at best buy, and i know we don't really profit from them that much because ours actually cover a ton of stuff. the best part of ours is the no lemon policy, so if we fix it 3 times, or if it wouldn't be cost effective for us to repair it, we give you what you initially paid for it to buy whatever new one you want. that way, you can still upgrade like you would have if you didn't get the service plan, but we pay for the new product.
i also have personal experience with circuit city's laptop service plans, and i was pleased with the service i recieved from them. my mom bought me the 2-year when she bought me my laptop for college, and it crapped out about 6 months ago. i called them, they sent me a box, i shipped it off, and i had it back and working like new in about a week.
don't get me wrong, some places DO have service plans that are ripoffs (sears, home depot, and lowes are not particularly impressive), and i personally think that best buy's are more comprehensive than circuit city's, but don't just assume that they're pure profit for us, because it couldn't be further from the truth (we give away a lot of free product every year because of them).
if what you're buying is something you're investing a good amount of money in, you might want to think about getting it. if it's not, then i wouldn't bother with it. $500 seems a bit steep, though, i'd go with their 2-year plan and then just renew it after it runs out if you think you might need it. i'm not sure, but i don't think ours are that much.