I have only experienced it once, but I can assure you, it give you one hell of a weird feeling. Usually, you wake up, still being a tad sleepy, but at that time, I woke up and felt very energetic, though I couldn't move. It was definitely scary, because no matter what I tried, I couldn't do anything but spin my eyes around, and because I had never experienced something like that before, I thought I got fully paralysed one way or another... Luckily all feelings returned after a couple of minutes (5 or so). I'd want to experience it another time though, so I wouldn't be scared and I would see what I am able to do while being paralysed (as in, which movements can I still make?).