We have had our wii since they first came out and haven't had a problem. jmjj - The sensor bar does have a thin cable that could easily break (however my 2 yr old has pulled it down numerous times and it still works). The kicker though is that the sensor bar costs about $10 to replace and the warranty for 3 yrs is $40. So if the cable does break and you replace it, you are still ahead $30.
Extended warranties are rarely worth it. Usually if it needs repair more than once, most people want to get rid of it anyway. Save your money and don't buy it.