the way i see it... ...something would have to go drastically wrong for you ever to get your moneys worth from this kind of extended warranty, i have a dell myself and haven't really encountered any problems with it however I am just out of warranty and one of the hard drives is reporting strange errors when i boot up but everything works fine - if i do have to replace the hard drive it might cost me £50 at most and I'll probably end up with a bigger one as well. These days I think the screen is probably the only thing that could go for it to be worth taking the extended warranty, if you look at the memory and processor a like for like replacement is going to be less than £50 (in most cases) once you're outside of the initial 1 year period. Sound cards are for nowt, and graphics cards need to be upgraded quite reguarlarly anyway even if you only play the latest games occasionally.
I have a Dell and so far( touch wood) no problems. A guy that I work with bought a top of the line machine from Dell about 2 and a half years ago, recently the monitor packed up, he phoned them up and asked about the replacement cost, got quoted some stupid figure and started looking online for cheaper options. Found something about half the price of the Dell option and ordered it, the next day Dell phoned him and knocked about a third off their original quote, still not cheaper than he had found on the net. Seems that if you don't take Dell up immediately on their offers they will get back to you to try and get a sale. Personally I would try and hang out for a better deal( i.e £20-£30) for the cover, if they don't expect their machine to go wrong then there should be no problem offering you even cheaper cover.