I use this all the time.
It's a normal stae the body enters at REM sleep. Waking up in this state while not common is not dangerous at all. However since you main chest muscles are paralyzed it feels as if something is on top of you. Also at this point you are still in REM state even if not asleep. You can hallucinate.
To get out of it I have two recommondations:
1. Keep you eyes closed ... think of a place you want to be.. You might end up there in a lucid dream. This is what I do and I induce sleep paralysis often for lucid dreams.
2. one thing you can control in SP is your breathing. Breathe deeply, then shallow... vary it often... in a few seconds your brain will realize you are awake and end the paralysis.
As for Astral Projection i have not tried it yet... but I have an open mind.