There was an interesting documentary series on the BBC called Getting High for God that had an episode on Ayahuasca. My take on it is that if you frame the experience as a spiritual journey, then you're most likely going to have a spiritual experience. Nothing is really happening beyond your brain processing what's around you and what's in your mind at the time though.
When I took LSD when I was still a JW I had Armageddon style trips, but later, when I had become an atheist that all stopped and I had trips where I felt more closely connected to nature and the world around me. Like I've said elsewhere, it's great for learning more about yourself, but you don't learn about anything that's outside your head.