Click'r pedals are meant to offer an easier release than standard Shimano SPD (i.e don't have to twist so hard).
SPD-compatible pedals like the Eggbeater or Time Atac use different cleats, but have the bolts in the same place so can be used on the same shoes. On some compatible pedals (eg Wellgo), the cleats are so similar to Shimano's that they are effectively interchangeable, on others (Speedplay Frog), they don't even look similar.
Different pedal systems offer different amounts of rotation before the clip starts to release the cleat ("play"). This rotation may either be completely free, or may have a return to centre spring force. More play allows less precise cleat positioning, and those with sensitive knees find them better. Once a cleat has been fitted to a shoe, it leaves indentations in the shoe sole that make it hard to adjust the position slightly.
Double-sided SPD with a cage (such as the M424) allow a good foot platform for pedalling before you've got clipped in, but aren't too good for riding with ordinary shoes as the clip mechanism stands proud of the cage and makes an uncomfortable bulge against the sole of a shoe without a cleat recess.
I'm currently using PD-M530 double-sided SPD on one bike, and PD-T780 (SPD side/flat side) on the other. The T780 mostly hang so I clip in just as fast as the M530.
The easiest to clip into that I've ever used were the Eggbeaters (but the original version had fragile bearings),
The easiest to release from I've ever used (that I didn't also pull out from) were Onza HO with well worn cleats (no longer available).