Shimano
Shimano is the only one of the bunch that has pedals made from metal, aluminum to be specific. This improves there durability, particularly across the platform that you cleat makes contact with. As the pedal and cleat wear you can get rocking so a more durable surface is desirable. The engagement is solid but the large button headed bolts limit the amount cleat adjustment. The Shimano 105 and Ultegra versions allow a little wider foot stance than do the Dura Ace so you need to have your feet a little wider. The alloy pedal bodies of the shimanos should stand up to crashes better than the composite of the other brands. Cleats are available with float or a fixed configuration.
Time
Times pedal design has a nice feel to the float and a solid engagement. The cleats are limited in side to side adjustment so you may not be able to get the proper stance width. To adjust width you swap cleats from your right shoe to the left. This give 2.5 mm of adjustment which isn't enough in my experience. The composite or carbon bodies are prone to braking off at the tip if you are a little fumble footed when clipping in. Cleat life is good as the actual engagement point on the cleat is metal.
Look
Looks Keo line engages well but the smaller cleat, compared to their older pedals, limit adjustment. With the Keo, I personally can't use them as I can't get my feet wide enough. Again these are a composite bodied pedal so over time the platform can wear leading to slop between the cleat and pedal. To combat this the have added a stainless steel plate but only on the more expensive Keo Max. Three different cleats are available with different amounts of float from no float to 9 degrees. The older Look Delta design pedals had more adjustment but were a little heavier with a slightly taller stack height.
Speedplay
Speedplay Zero and X pedals are unique to the bunch in that they have the retention mechanism on the cleat rather than the pedal. Of the pedals the Speedplay is the most adjustable as there are different length axles available and different cleat mounting plates so there is lots of adjustment. This comes at a price though as different axles and adapter plates must be purchased separately from the pedals, increasing the cost quite a bit. The float of the pedals is free, so initially it feels like your standing on an ice cube. This goes away after you've ridden them for a while but some people won't like the feel, myself included. Speedplay pedals are the most aerodynamic so you can save a bit energy wise. Speedplay cleats are the most expensive of the brands mentioned as the retention mechanism is in the cleat.