Personally I'd stick with flat pedals for anything less than 20 or 25 miles each way and even then I might stick with flat. Then again I don't wear any special cycling clothing for commuting or transportation but regular shoes, slacks, and shirt.

That said, my favorite pedal for other than training/racing was Time ATAC. Easy to get clipped in and out but still worked fairly well. The cleat is small and low profile so allowed easy and quiet walking in a wide variety of shoes. I wouldn't race in them (I prefer Zero's) but for lower intensity riding they're great.