Most cleats have a few degrees of "float" where your foot is allowed to swivel laterally. Speedplay pedals probably have the most and require getting use too. I bought after market market cleats for my DuraAce road pedals that have extra float.
Many MTB riders have gone to a pedals that look like a fat BMX pedal that can both lock you in, or allow free-riding. You'll see bike trials riders with those.

I was racing back when pedals changed from clips to clipless. There was a huge difference in comfort. First, you don't have a strap biting into the top of your foot when you're hammering up a big hill or trying to sprint. Second, it's a lighter system and much more adjustable, allowing your entire shoe to be the pedal base.

Clipless are also much safer for MTB riding. I have a 4" scar on my shin from when I stalled on a hill, yanked out my foot from the clip, and the quill came around a sliced my shin. (back in the stone ages when MTB's came with clip pedals) You can bunny hop 100% better with clipless too, road or off road. Fly right over those rail road tracks! <img src="/forums/images/graemlins/grin.gif" alt="" />