I use Speedplay Frogs on the road and will eventually use them on the mountain bike. I've never had a problem with them unclipping accidentally and with a lot of float they are quite knee-friendly. There appear to be a lot of competition amongst mountain bike pedals, and Speedplay is releasing a new mtb pedal this year with adjustable float - though they will still make Frogs & have no intention on ending the line and forcing people to choose another pedal.

Frogs have no tension, so it is simply about heel placement to get in/out. I was afraid when I switched from SPD that I would have a problem twisting my ankle far enough but it's never been an issue. My fitter strongly advised me against both Time and Candies because of my size and ankle issues - he thought that I would likely get hurt using them because they can, apparently, be difficult to get out of.