I read a thread about this here on GAF last summer, thought cool I´d like to experience that. I did later that week. Since then it's been happening a few times every month or so.
Being aware of this phenomena helps you recognize and remember it. After a couple of months I started to remember SP episodes from long ago, so this had happened to me before. I had probably discarded the experience as dreams.
This happens to me regardless of sleeping position, and I´m not typically bothered about being paralysed, cause I don't bother trying to move. Being aware while falling asleep can be scary and I used to panic and try to wake back up. When entering SP from a dream there's usually a sudden climax of lucidity in the dream leading up to the SP. For me there's also a big difference in quality between SP episodes. Some are more dreamlike and some are more wakelike.
I don't believe you see your actual surroundings and hallucinate, I think the whole experience is a hallucination in the same way dreams are. Although my room feels and looks absolutely real during SP, paying attention to details tells me it's not. There are usually differences in color and positioning of objects.
I mostly enjoy SP but it can be terrifying. A few months back I woke into SP facing my wall which was scary, cause I´d like to have a better view of the room. I thought I'll just close my eyes and go back to sleep. Then someone opened the door into my room(I live alone), slamming the handle down. When I heard that I was like: Wow, that sounded pretty real. Then I heard it again echoing in my head (see:
http://en.wikipedia.org/wiki/Echoic_memory) and I realized that even though I know this is just SP the experience is real right now. Then I felt the absolute terror washing over me. I woke up seconds later.
I think audio is one of the things that define SP, in contrast to dreams audio in SP feels absolutely real, you hear things, outside of your head. You also feel things. I usually feel like there's an intense vibration going through everything, probably similar to what some of you describe as a sensation of electricity.
There's a relation between SP and lucid dreaming. Read this wiki article:
http://en.wikipedia.org/wiki/Lucid_dreaming#Wake-initiation_of_lucid_dreams_.28WILD.29 Wake Induced Lucid Dream. It's a technique where you practice falling asleep while maintaining awareness, going through the stage of SP to enter a lucid dream.