Living in the northwest I wouldn't use the Shimano's for mountain biking because I ride year-round, and it gets difficult to clip back in with mud packed in your cleat. With the CB pedals you can jam your foot down and the mud drops through.

I've got the regular eggbeaters, and love them. I'm able to get in and out easily.

Look has a mountain bike pedal too that has an interesting design. Pretty open, like the eggbeaters. I'm interested, but not interested enough to start trying out new pedals right now.