I love the Untapped Cities newsletter--full of fascinating history and, indeed, secrets about New York. Since I grew up in New York but no longer live their, it's particularly fun for me.
Consequently, when planning activities for me, my wife, and a couple of friends visiting from Australia, an Untapped Cities tour appealed. Though most fascinated by the idea of the subway tour, I chose the Brooklyn Bridge because it would be outdoors.
The experience was great--it would be great doing it unguided, but our guide was informative, enthusiastic, knowledgeable, and personable, and added a great deal of value.
I had hoped for a little more "secret" content, but I have read David McCollough's terrific book "The Great Bridge," so perhaps that's unfair.
I'd definitely recommend this tour.