I'm a long time LOOK user. Used both the Keo and the Deltas. They're a great pedal system and possibly the easiest/quickest to get going with from a stop. They have good adjustability. I am very happy with them and have gotten several sets over three bikes now, currently using Keos. I'm very confident and comfortable in them. SPD-SLs would also be a good choice from what I understand, but nothing has really encouraged me to switch.
Walking in the LOOKs, if and when you ever do, requires the cleat covers to be on and then its no problem at all. Dont leave home without the clovers.
You CAN walk in them without, but you have to favor the shoe heels and you will wear down the cleats uneccessarily. SPD-SLs have a leg up on LOOKs in that department, but I dont walk in my shoes enough to have really thought about it. Probably the SPD-SLs with the rubber parts come down onto the road with more forgiveness if you come down onto the cleat, but I come down on the heel and would obviously encourage that with any cleat system, but this too might be looked at as an SPD-SL slight edge. LOOK cleats do wear faster, but if the only real road touches they get are the ins and outs coming off the pedal, its not too big an issue.
All said and done, both are excellent pedal systems, but capwater summed up my sentiment.
