The coffee tours on the Big Island aren't real tours. They usually walk you a few feet to a coffee bean tree, show you a cherry and bean, show you the pits and dryers, and then off to the gift stand with some free samples. It takes about 10-15 minutes.
Bay View Farms and Greenwell have bigger operations and good vista views. Otherwise there are about a 100 small farms off the various roads in South Kona that will give you a tour.
Is it worth it? It's a short stop if you are in the area (going to Captain Cook's, Place of Refuge, or toward VNP), so I would totally do at least one.
I wouldn't drive all the way from Waikoloa for to visit a coffee farm, but there is tons to do and see in that South Kona area.