We hired one for both of our girls, and would do it again. I have thoughts:
1) This is an arms race caused by "elite overproduction." It's stupid, unfair, and much of it is arbitrary. Essentially, the process is akin to a cold resume coming into an HR team for a limited number of posted positions. They can say it's about finding the right fit and no doubt they would like it to be, but they are drowning in qualified applicants. Outside of the very top of the very top tier, there's a little luck and a lot of volume involved. So network and expertise matters a lot, enter the role of the counselor.
2) Junior/Senior year is very late to start the process. Ideally you would start the kid with a counselor before the beginning of freshman year.
3) The counselor is not there to do work for your kid or pull strings. They exist to navigate the process from an expert position and help your kid focus their efforts on things that matter, while giving them supportive but 100% honest feedback on where they are, what they want, and their chances. For that reason counselors should be chosen based on their track record, career history, and how they structure their team. Interview them, ask for refrences, etc.
4 ) The organizational element is where counselors really earn their keep - we're talking about 15-20 applications, many of which will have their own essays or are looking for different things. Counselors keep all the gears turning smoothly in the process, including getting on the students when they are procratinating, @troph. When I said "team" earlier, I meant it. Larger practices will typically have a number of coaches who are managing the process, to go with tutors and essay coaches/editors on staff. Division of labor is the sign of a strong practice.
5) Frankly, I hate this game. It's a form of opportunity hoarding that systematically excludes kids from families without money/savvy/interest from access to ladders of opportunity. That said, we have all three and deployed them for our daughters, with good results.