I think that science and experience has established that truth serum does indeed work. While it is not something that works as it is portrayed in movies, there are drugs that make a person much more amenable to coercion and as such are more likely to tell the truth to someone.
Truth serum is still a developing science. The beliefs about where truth in the brain comes from change too often for us to have figured out how to make people always tell the truth. It is too early for us to depend on truth serum for serious purposes like justice and interrogation.
I think that people who believe that truth serum is a real thing have essentially been watching far too much science fiction or Hollywood movies. Such a compound does not exist, nor would it be a moral thing to do if someone were to use it on someone else, I say.
There is not enough evidence to say that truth serum will work in every case. Sometimes it may, as all it does is lower a persons inhibitions, much like alcohol. The problem is that you do not know what to expect and the person in question may not be truthful, even after receiving a dose.