There are actually 2 types of SPD cleats, on the bottom the other type being stamped with a large M for multirelease which allows them to disengage with upward pull. Not so keen on them myself having fitted a set by accident (then having one) when I purchased some new boots and pedals with some birthday money. I fitted the new pedals to one of my other bikes, installed the cleats onto my new shoes, adjusted spring tension and had a test ride round the block, perfect. Then I decided to go for a spin to a country pub I like, first hill I got to I stood up to climb, my foot shot out of the pedal, knee hit the bars and I hit the deck. Needless to say they came off pretty sharpish and now reside somewhere in the back of the shed.
Having got used to SPD several years before and having my fair share of" moments" (falling over at home after unclipping and clipping back in for hours on a ride seems to be very popular online) I can only say whilst they may be of use in some cases e.g. mountain bikers who need to get out in a hurry,I would advise riders to be aware of this.
Also a trick when using single sided pedals (to clip in) is to remove the unclipped foot from the pedal at the bottom of the stroke whilst still pedalling with the other then when the pedal gets to the top it will be the right way up due to centrifugal (centripetal) force and you can engage the cleat easy peasey.
