I have had one or two near misses, but never fallen. I pretty much only clip in my right at the start of my ride and unclip at the end so I am a little less practiced at that side, whereas I always unclip my left when I stop (to stand on). If I am stopped and somehow lean right, it can be a hairy split second to get unclipped to avoid embarassment tho.
Start off with the loosest settings (if you can adjust them) and if you panic, you will be able to get your foot loose.
I did spend an evening sat on my bike practicing clipping and unclipping before attempting going out, and probably unclip my left way too early when I know I am likely to stop, or slow to a crawl.
I started off with double sided SPD's (which can be adjusted) and now have Egg Beaters (which can't). I haven't looked back since getting them. Not literally, obviously, that would be dangerous.