Obviously hard to tell how accurate this is... but according to a survey on 360gamer.com, 59% of Xbox360 owners have had at least one unit fail on them. 21% have had more than one fail.
My 360 isn't seeing much action these days, but I know that every time I power it on I'm just waiting for something bad to happen. First and last time I've ever bought an extended warranty.
How about here... has your 360 failed?