The discussion of "Ātman is Brahman" and "That Thou Art" (sometimes for clarity written, as above, as "That One, Thou Art") is an objective discussion based in Vedantic theology as expressed in the Upanishads (the theological discussion portion of each of the four Vedas). The questions of "appeal" and "repulsion," however, produce thoroughly subjective discussions based in an individual's philosophy, religion, theology, worldview, and personal sentiments; these may have no realistic bearing on the objective discussion of Hindu questions of Self and God as expressed in the sayings above. Finding the answers to the questions of what you feel (appeal or repulsion) and see (truth or non-truth) will be a quest of exploration through your own inner being, although the quest may be aided by an objective discussion of Ātman and "That Thou Art."
Brahman is the name of the infinite consciousness that is infinite existence and infinite bliss (Johnson and McGee). Ātman is the expansion of this Brahmanic essence, which has manifest as the essence of humanity and as a repository of that which is pure, without limitation, and perfect: it is Brahman unborn and undying within each human; it is Brahman appearing in time and space (Johnson and McGee). Consequently, the teaching that Ātman is Brahman is the logical extension of Vedantic teachings and is sound.
The "Ātman" supersedes all physical limitations of human beings; being the real consciousness and perception, Ātman is limitless and uncreated. Ātman, the non-corporeal seat of true thought and perception, employs human characteristics as "instruments" for living in a corporeal world (Swami Nikhilananda, "That Thou Art"). "That Thou Art" designates that unknown yet somehow known inner being—each person's inner "thou"—as the seat of oneness with Brahman in-dwelling. Recognition of the Ātman's connected oneness with Brahman emanates from a person's extension beyond the limiting physical realm into transcendent knowledge into true knowledge. In this light, the saying "That Thou Art" is a correct expression of Vedantic teachings on God and Self—on Brahman (God) and Ātman (God dwelling within).