C C C:
Why not try reading that Castaneda stuff and make up your own mind?
I read a ton of Castaneda as a kid. My local library had the whole set (right next to the "Time Life Mysteries" series and the Eck books - loved those too.) Finding out that he - and the Eck guys - were frauds was a big blow to my 12-year-old self. I think I started thinking of myself as an atheist (a materialist atheist, not the Buddhist kind) soon after that.
The fact is Castaneda didn't just fabricate some of his stuff. That alone would be disturbing enough. But the truth is far worse: It was basically all made up. All of the more interesting stuff in his books, the Inorganic Beings, the Focal Point, the Universe being a (somewhat predatory) female in search of the rare male essence etc. bear little to no resemblance to the actual beliefs of the Indian tribe he supposedly lived with, when he was actually in LA writing science fiction for New Age suckers to buy as fact. Castaneda learned well from L. Ron Hubbard's example and I consider them of the same ilk.
It's hard to let go of all the fantastic horizons we probably all saw as kids reading these books. Very hard. I don't think I've ever fully let go of them even to this day. There's always the glimmer of hope that somewhere out there another dimension, another reality, really exists and that somehow we can find it and this will cure our suffering. But as this is a Buddhist board I will point out that in Buddhism no experience, no matter how fantastic, is a cure for suffering. Even if Castaneda's world was real, eventually you would get bored with it. Only the letting go of experience is a cure for suffering.