I slipped on some streetcar tracks last Monday (front wheel slipped out from under me) and went down, when I hit the floor and started laughing at my sillyness over the tracks, I realised I didn't even have to think about unclipping, it just happened - the bike was already on the floor as I was trying to break my own fall. I wouldn't say it's instinctive for me, but I seem to manage - the times where I've panicked, my tensioners are loose enough that a frantic foot waggle will get me out - though I've been VERY close, I've only had one semi-fall (foot landing, but only just) and that was when pratting about showing off the SPDs to someone in their yard