Now, this isn’t scientific data, especially since we know that Microsoft hides that data from prying eyes, but it’s more than supposition. Game Informer did a poll and found that the 360 has the highest failure rate of any of the consoles at 54% — not exactly the most unexpected news, but putting a number on it is kind of sobering. The survey (which is print-only or I’d link it) also notes that the 360 gets a bit more playtime than its competitors, which might account for a bit more failure, but the PS3 was failing at 11% and the Wii at 7%.
You guys may remember this excellent article describing the process of creating the 360 and the drive to get it to market. I’m guessing that at that time, the failure rate was probably as high as 75% past a certain level of play. But as the design was refined and the manufacturing improved, I’d put it (entirely guesstimation here) around 20% or even lower.
The question probably asked whether a console has failed on you, and if so, which one. In that case, practically anybody who seriously used their 360 before a year ago would be responding yes.