I've heard mountain bikers and cyclocrossers say that there is more of a tendency for your foot to roll off the pedal while trying to clip in, in a hasty fashion.
I have not used them but I have used Time Atac and don't like them.
It's the feel of clipping in and out that I don't like.
They do not have the positive tactile feeling that Shimano SPD's have.
Also, SPD's have adjustable entry/release tension. You can make them super easy to clip in and out of or clamp your feet in tight for fast riding.
Last edited by GeoKrpan; 02-21-07 at 11:10 PM.