while i have no direct comparative experience with the shimano mtb pedals, ive been using the aliums for about a month now, i definitely like them.

solid engagement, ive never come unclipped when i didnt want to. nothing to adjust unless you want to change the unclip angle from 13-17º (which simply means reversing which cleat goes on which shoe)

while i prefer my shimano road cleats for most rides because they cut down on the float, im definitely a believer in mtb pedals and shoes now.

i have a set of issi pedals which are similar to the shimano mtb pedals ill be trying out once my latest project is complete so i guess i could give better input then

TL;DR

they are good. i like them. you should buy some.

look on ebay, the newer pedals are more expensive and i snagged a barely used set with a barely used set of shoes for less than $100