I recently went to proper SPD SL from mountain bike SPDs.
To be honest, I wouldnt have liked them if I hadnt got used to normal SPDs first.
One of the main differences is that if you miss the clip in with spds you can quite easily pedal a few turns with the pedal under the arch of your foot as the rubber sole will grip the pedals. With SPD-SL if you try that you will slip off the pedal. If you think you are cliped in but have missed and go to "Give it some" you will slip off, but only after you have raised yousrlf and inch or 2 out of the saddle to drop back down on the jewels!
Nobody told me that.