I really am not a fan of heights, but believe it or not I used to work at a place where they do this kind of thing. It was pretty interesting. The course was only about 20 feet in the air, but we had to practice rescuing people and stuff like that. The equipment is all really sturdy and that actually gives you some level of comfort even if you don't enjoy heights. The craziest part for me was when you had to clip the person you were rescuing onto your own harness then cut their line. Talk about scary!
I can do high ropes quite easily, so maybe I could do this too. Climbing over the edge at the top is the worst looking part. As you say a harness gives you that comfort zone.
Yeah, being at the top would probably be pretty bad. Especially if you tried to do it like they do in the movies where they walk down the building facing towards the ground. I don't think I could do that harness or not. It's always a little less stressful when they have you on live belay too.