The work Dr. Thomas Szasz's did was absolutely legitimate. There is truth in the fact that mental illness could easily be redefined as simply the way a person acts. Normal doesn't really exist, so its difficult to base mental illness against anything or say that anything is particularly wrong. I think his theories are interesting and they certainly fall in line with what others sometimes think, so there's no reason that it shouldn't be considered legitimate.
Dr. Thomas Szasz purported that many mental illnesses are faked simply because they cannot be detected by physical means. One of the few exceptions to Szasz is Alheimer's disease. Mainstream psychiatry refutes Szasz's claims that the DSM is faulty and there are no scientific proofs for mental illnness. However, that doesn't mean Szasz was full of bad ideas. Sometimes a psychosis really is just a fake shield to hide behind.
Psychology isn't a hard science for very important reasons. The most prominent reason is that there are no definitive answers as to what causes psychoses and other psychological problems. Dr. Thomas Szasz maintained many psychological problems simply don't exist and are "made up" because hard science can't detect them. Mainstream psychiatry doubted his findings and research. However, science needs doubters in order to survive. Although shunned by mainstream scientists, Szasz did have legitimate complaints against psychiatrists and the DSM.
After doing a little bit of research on Mr. Thomas Szasz, I do think his work appears to be the real deal. He seemed to have a very legit point, and very political when it comes to doing his very best to get his point across to the national public.
The questions that Dr. Szasz sought to answer with his work were ones that deserved an answer. The primary question, is mental illness a real disease? However, Szasz had already decided the answer to the primary question was "no" and his research was completely biased and selected to support the predetermined answer.