Sure they will clean up along with some stuff you dont want them to clean. There are so many species of Urchin, you have to specify. Some are ok in a reef and some are horrible.......
i have had a few thrive in my tanks before. but i had to remove them because they constantly knock things over. the kind i have owned also eat my coraline algae =(
yeah i agree w/steve. they wedge themselves between rocks and rearranage them. they also leave a trail of 'mowed' corralline algae where they've been. imho the possible risks outweigh the cost/how cool they look. there are many diff species but i believe these risks are present in all...