Casey - We didn't use one at all, so I can't really comment on how effective they are.
What I know for sure is that the college coaches will want to see and evaluate your son themselves if at all possible. I kind of doubt they'll leave it to someone else's evaluation unless they know them well - and usually thats a scout or another coach they trust. So it is very important that you get your son in front of them via a reputable showcase or summer program. You don't have to look very far on this website to pick out a few showcases that parents have had good experiences with.