My elementary school had them (though that was some time ago). My last year in elementary, I was one myself.
I had a couple bad experiences with them in 4th and 5th grade, so when I came to be one, I mostly just issued warnings. I made sure the other hall monitors weren't being dicks as well (I was the captain - yes, I earned my spaz card early in life).
I've always believed that those who seek power, do so for selfish reasons, and that power ultimately corrupts. Same with police, politicians, etc. If I had been one for more than just that year, I don't doubt that I might have become a dick too.