Vilna Gaon on learning kabalah:
He that is able to understand secrets of the Torah and does not try to understand them will be judged harshly, may G‑d have mercy. (Even Shlema 8:24)
The Redemption will only come about through learning Torah, and the essence of the Redemption depends upon learning Kabbalah. (Even Shlema 11:3)
All those who don’t understand the secret meaning [behind something], do not grasp even the simple meaning. (commentary on Proverbs 5:18)
The Evil Urge is unable to overcome all who occupy themselves in the hinting and secret [levels of Torah]. (Ibid.)