I run several sets SPD pedals: 520, 540, 324,and A520.

I tend to agree with Staehpj1. The 520 type SPD pedals are what I have used for the last 3-4 tours. I can pedal for short distances wearing my running shoes without any problem. I also like the double sided clips, especially when starting, and clipping in on steep hills.

If you look at any SPD pedal with the shoe clipped in, you will see that the only contact with the pedal is at the cleat, regardless of shoe or pedal type. The sole of the shoe does not contact the pedal. The A 520 is the road version of the mountain 520 with a cage around the clip. With my road shoes on, the cage is just a decoration. I think my wife wore her mountain bike shoes riding her road bike with the A520 pedals a few weeks ago. Even with Mtn. bike shoes the soles of her shoes did not contact the cage. The shoes and pedals would have to be pretty closely matched to be able to clip in to have the sole of the shoe have significant contact with the cage.

The 324 pedals just beat the heck out of my shins when walking the bike or loading it on a train etc. There are a lot of sharp, pointy places on those pedals.