I use clipless on my mountain bike. I used to be petrified of not getting out in time, so I would keep one foot unclipped on the downhills and really steep uphills just in case. But I worked and worked at it. I realized today as I was tearing down some rocky twisty downhill with switchbacks that I stay clipped the whole way-- they help me maneuver better and they even kept me on my bike when I should have bounced off (oh, for full suspension...)