This novel and its accompanying movie are relevant today just as Cool Hand Luke is yet relevant. The maverick who refuses to conform and be defeated by just following the status quo is, indeed, a motif that is relevant today. McMurphy is irrepressible--a hero to anyone who has had to be subservient and fawning.
I certainly think that some of Kesey's work might seem outdated. Advances have been made in the field of understanding mental illness and treatment of such conditions in a manner that makes the ward of Nurse Ratched (thankfully) something of the past. However, I do believe that in a more subtle manner, there are many parallels between the manner of Nurse Ratched and the entire configuration that surrounds the individual. Nurse Ratched never raises her voice and never outwardly does anything that could be seen as tyrannical, yet she controls the patients and the ward with an iron clad sense of control that belies her calm exterior. This could be something that can be seen on more personal levels. For example, the relationships between adolescents in which one member of the group exerts a dominant control over others while not doing anything outwardly hostile. The social dynamics that happen between adolescents is one where there is definite relationship where one person is in control and the others must either face expulsion or comply. This element is present in the manner that Nurse Ratched "works" with the inmates. These social dynamics compel the individual to either surrender to the dominant force or resist and make a sacrifice. This is where greater parallel can be found in Kesey's work. Kesey is suggesting that when individuals are placed in a situation between an irrepressible and controlling Status Quo and the desire to be free, some sacrifice, however uncomfortable, must be made. When adolescents are in such situations with individuals who are so dominant or so in control that individual happiness is threatened, some need to "break free" and sacrifice is warranted.