It's something I got greatly interested in lately, having a few projections myself. I know the experience is real, but I do not yet know if it is all in the brain or if it is a genuine leaving of the body.
http://en.wikipedia.org/wiki/Astral_projection
The evidence I have seen does lean towards it being an actual experience, but I cannot say for sure yet. My friend and I will be trying to meet each other out-of-body atleast 20 times, and say a 5 digit number to each other. We will then repeat the number to each other, digit by digit, to see if the experience was true. ( I saw the first digit, he says second, I say third, he says fourth, I say fifth. )