I'm glad I did, for Kahana Valley is a place of wonders. The hike begins with about a mile or so of road walking. Once that is out of the way, you'll find yourself wandering through a valley that has yet to be ravaged by developers, dirt bikers and the like.
The trail I took consisted of an eight-mile double loop that crossed the Kahana Stream several times. Other trails exist, though, and following them can take you to spots deep in the valley, including a trail that leads to a dangerous hike up Pu'u Ohulehule, the pyramid-shaped peak in the far left hand corner.
I was amazed by the abundance of hala trees along the trail. In fact, the path you'll traverse is carpeted with hala leaves for much of way. As I hiked, I had visions of ancient Hawaiians gathering the hala leaves for mats, sails, and other items they used.
Beware of the mosquitoes!! When I stopped to take a swig of water, a sqadron of the pests had swarmed on my legs. Thereafter, I kept moving, stopping only when I needed to glance at my trail guide. By the way, I'd recommend taking along Stuart Ball's book (see the intro for more info on it), for there are several key junctions where one could go astray.
If you want to explore a huge, undeveloped valley with a gently flowing river that has several nice swimming holes, try the Kahana hike.