First of all, booking this tour was a breeze. We just called the number, made a reservation, and we were in.
EVAN, our guide on the day of the tour, was AWESOME. He was super fun and knowledgable about all kinds of interesting tidbits along the tour, like local vegetation, history of the area, etc. My wife and I agreed that if there was a poster boy for a fun tour guide on Kauai, Evan would've been it.
It's about a 2.4 mile kayak trip up the river followed by a 3 mile hike to Secret Falls (which, judging by how many people poured into the place, aren't all that "secret"). Then back into the kayaks and back down the river you go the way from whence you came.
My suggestion would be to either book the earliest tour you can get, or the last of the day, (leaving around 1:30pm), as otherwise the crowds at Secret Falls are a pain. When we got there around 10am, about 6-8 people milled about, and by the time we were done picnicking (only 35 minutes later), there must've been more than 50.
Honestly, having a awesome guide on this tour makes all the difference, so if you have a chance to hang with Evan, dew it!!