I was amazed at how well a little bit of consistent practice could bring out this ability. I don't recall any negative side effects as others here have mentioned, but then I was a teenager so who knows if I was really tuned-in to how it was effecting me.
I remember being especially entranced by the idea of tests subjects sending coded signals to the waking world through muscular contractions - but then Dreamscape was a relatively new film at the time, so maybe that had something to do with it.