Kabbalah, or Jewish mysticism, should be more mainstream and more common as Jews modernize their beliefs. Every religion has a place for a personal relationship with deities and Kabbalah is one way to foster that relationship with Jews. Getting to know the spirit of G-d brings meaning to the Torah and Talmud, so Kabbalah should be a legitimate part of Judaism in order to foster personal growth throughout a lifetime.
Every religion has an outward set of practices for most adherents and another set of internal, more mystical practices and doctrines for those who want to go deeper. In Judaism, Kabbalah falls into the second category. It is not only legitimate but an advanced area of the Jewish tradition and should be highly valued.
Yes, I do think that this is a legitimate part of Judaism. The reason I believe this is because the people who practice this religion want this to be a real part of Judaism. Since they believe in it, who am i to say that this is not part of it.
While people who practice Kabbalah are practicing tenets associated with Judaism, it is actually separate from Judaism itself, so I believe associating it directly is a false statement. Therefore it can not be a legitimate part of Judaism, because it is not a part of Judaism. There are plenty of people who follow Judaism who don't particularly believe in Kabbalah.
There is nothing serious or legitimate about Kabbalah. It's the Scientology of Judaism. It's nothing more than a way to part celebrities and other people with lots of money and not enough sense by promising them a lifestyle they can buy. This was evident in Madonna's brief fascination with the sect.