I have also heard testimonies like this. Some were not so pleasurable.
Some appear to visit hell. One story I remember goes something like: a man after visiting hell was revived and then devoted his life to studying the Bible and became a minister.
As far as evidence of afterlife experiences, I think you would have to look at the individual cases and verify that the details of activities that they can describe fit what actually happened and that there is no other way they would know. I think this has been done to a certain degree, but I haven't reviewed the evidence myself.
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