(No joke, the monkeys were that close)
Well, I was most certainly not disappointed by the experience, and I clearly lived to tell the tale. My most succinct reaction: I think it is actually the closest I will ever get to being a monkey - well, a 5'8" monkey with no grace or coordination. The tour took us through the top of the rainforest canopy through a set of zip lines and platforms. Coordination wasn't even a requirement. Ha - I can still see one of the tiny little tour guides cringe as I came barreling toward the platform. I will admit that at one point I felt a twinge of vertigo, but that passed quickly as my carabiner was swiftly strapped into the next rope, and I was pushed down toward a platform I could roughly make out in the canopy of the next tree.
I would say if you are even the least bit adventurous, then it is an experience that should not be missed. Sure, you can absorb some of the nature from a hike or a horseback ride, but the perspective and view from the tippity top of a tree CANNOT be beat.
(I was apparently too scared to take a picture of anything except birds - boo me).
Tour providers are really easy to find. We stayed on the western coast near the town of Jaco (get surf town - that's another story), and we probably drove past over a dozen different signs for companies offering zip tours. If you are wary of safety, then I would recommend simply asking your hotel for recommendations, or going to any tourist gift shop will also have brochures.