A good book on the subject is Proof of Heaven by Eben Alexander.
He is a neurosurgeon and was not a believer of NDEs - until he had one.
His experience was exceptional in that they had him hooked up to every monitoring device imaginable and zero brain activity was proven - and yet he was off somewhere else having an experience he describes as hyper reality - more real than what we currently experience.
I for one believe them to be real experiences - their are just too many of these NDEs happening for it to be easily dismissed.