Christopher Brian Colohan wrote in
:
For a casual cyclist: SPDs are the cheapest option, they work quite
well, and you can get them with a variety of cycling shoes which _you
can walk in_. The "I can walk in my shoes!" argument can trump
everything else if you are just using the bike to get around...

Any mountain bike clipless pedal system can be easily walked in. The biggest
advantage of Shimano SPD is that you get pretty good quality for a low price.
Some other systems offer better performance (especially in muddy conditions)
or lower weight, but always at a higher price.