Of the guys I ride with...we all use either SPD, SPD SL or Speedplay. I've never used Speedplay but I can 100% say, I've heard they are hard to walk in and pretty darn slippery on a smooth surface while watching my riding buddies complain about it in real time.

Personally...I actually found SPD-SL much easier to walk in the SPD. The two rubber feet on the rear of the cleat (SL) offer a "tripod" with the single heel point. Once you get used to having your feet a bit turned up...I found walking to be extremely easy (I mean it). On the other hand, SPD is extremely small and when I put that cleat on a pair of road shoes, I actually found myself rolling my ankles often if I didn't pay close attention. Unlike the tripod of the SL, all you have is your heel...and a tiny metal cleat dead center of your foot...rolling an ankle is easy. In a pair of MTB shoes, the SPD isn't as bad...but MTN shoes are more "flexy", far less vented and as others have noted...heavy. Add to that the fact that SPD only pedals are limited and heavy...once you go to SPD-SL, you're options are much better.

Oh...and one other thing...a SPD cleat will grind the crap out of stuff if you're not careful.

I guess it boils down to what you want. For performance...I strongly feel there is better options for road riding than SPD. The strengths of SPD are much better seen in off road riding. They are lighter, offer better pedal contact, lighter pedals and most of the high end shoes won't even accept SPD cleats. For "walk-ability"...SPD and a set of recessed "touring" or MTB shoes are better. They won't perform as well...but they fit a more "normal" use.

When I ride my bike...that's all I'm doing (save taking a pee or whatever)...I'm not stopping by the store unless it's after a ride and then I have my regular shoes on at that point.