Dukes was recommended by a concierge we used while on the Island and the experience was fun. This is not a trip for the sedentary or timid, however, and they don't tell you that when you book. We took an afternoon trip w/Evan as our guide. He was friendly, helpful, knowledgable and added a lot to the trip.
The kayak paddle is 2 miles each way up the Wailua River. The 3 mile hike to a waterfall was treacherous on the day we went. It was really muddy the whole way. Be sure to wear appropriate shoes and loads of bug repellent. Our hike was scary for some (lots of slipping and sliding and some falling in the mud) and many people turned around. Our group was small (6 people) and I suggest going with as few as possible as you have to wait for the slowest people in the group.
The journey is way better than the payoff as at the end of the trail you see Secret Falls. It doesn't feel very secret as there are lots of other people on tours at the same place. And Kauai is not short on waterfalls, so it isn't all that special.
We did, however, want the kayak trip and this was perfect. In November, the sea kayaking off of Na Pali is really dangerous, so the river was a great alternative.