It truthfully depends on the person. I've never thoroughly researched the subject, but I have made myself a lab rat to it, and on countless occassions would strive to beat my own record at staying up. I was always fucked up, the sleep dep just added to it.
But I do know people, and have heard other cases, where people have been hospitalized, and yes, died, from lack of sleep. Though, in the cases where they died, it was probably from a lack of nutrients and sufficient energy sources as well.