I used to use the older Looks until Shimano came out with their Look licensed 7410s. Then I used those until the SPD-SLs came out.

The Shimano cleat design is superior because it has a lower stack height, and there are rubber bumpers on the corners. That makes them safer to walk on. Since the walking wear surfaces don't have anything to do with cleat retention its ok for them to wear down. While with the Look cleats, the rear of the cleat wears when you walk on them, and thats what holds the cleat onto the pedal. I don't walk around in my cycling shoes much but when you are riding centuries there's always some bit of walking involved.

The SPD-SLs are slightly more difficult to get in to. The "window" in the pedal for sticking the front of the cleat in is smaller. That's the one disadvantage vs the Look pedals.

The SPD-SLs don't have the dreaded Look squeak.