Many years ago I rode with leather shoe clips and noticed some improvement after adding them, I decided to try bike shoes with clipless pedals, beings its a mountain bike I went with shoes and pedals that were beater style pedals.
here's a pic
Took a while to master the clips but they seem to be easier every time I ride.
The first ride it felt like I was pumping water with my pedals smooth but with resistance.
second ride I dropped the bike in soft sand plowed the front wheel and went down left leg came loose right did after on the ground.. at leat it was soft..
Third day much better ride seemed more comfortable at higher speeds and larger gears,learned to release shoes if approaching any intersection as a precaution.
I think they are great if you don't have to stop alot, I normally don't except when starting my ride then I just use the pedals without engaging the clips.
One big plus is they seem to improve the angle of attack on the pedals this agrees with my knees. when I didn't have them I was moving my feet all over the pedals sometimes my heel would strike the frame.