From a fitter's perspective who sees all of these on a regular basis (and is not a dealer for any of them as to not be biased):
Looks - least amount of cleat adjustment, and it's hard to make any cleat adjustment without affecting other cleat adjustments. The gray cleats only have 4.5 degrees of float and people with knee issues have a hard time with that. Low maintenance. Single-sided entry, not the easiest to get in and out of. A good 70%+ of my customers go with some flavor of Look KEOs. I honestly don't know why, other than peer pressure and local bike shops pushing everybody towards them.
Shimano - better than the Looks for cleat adjustment, but still similar problems due to the similar cleat design. Also low maintenance, single sided, and somewhat difficult for newer or smaller cyclists to get in and out of them.
Speedplay - the most adjustable as far as cleat position goes, by far. All movements are completely independent of each other, and their base extender plate kit can allow a much more rearward cleat position. The stainless versions are available in different spindle lengths, so Two-sided entry, and I like the light actions for newer or lighter cyclists. They require a bit of maintenance, and due to the open nature of the cleat, you need to be careful about walking in stuff that might get stuck in the cleat and not allow you to engage the pedal. They're a little bit more sensitive to being dirty.
Travis Rassat
Vector Cycle Works Noblesville, IN
BikeFit Instructor | FMS | F.I.S.T. | IBFI
Toughman Triathlon Series Ambassador