We went several years ago and its is truly worth the visit. Close to Negril so you don't spend an entire day on the road. You walk up through the falls and they are much smaller and tamer than Dunn's River or Y's Falls. There are rock slides naturally formed in the river that the guide can show you how to go under water in one place and come up somewhere else, quickly though! The water is cold and on a hot day it is truly an enjoyable experience.
If you're planning to do the Mayfield via CSA, make sure you find another couple, they will only have the excursion if they get 2 couples or more, we just got back last week and was on the list for the day before we headed home and no one signed up but us for that day so it didn't happen. The YS/Black River falls excursion seems to be more popular so you may get a spot in there if Mayfiled doesn't work out, we just prefer Mayfield because it's a bit quieter. Good luck.
Mayfield falls is well worth your time. It's not far from the resort. The surroundings are breathtaking and so is the water. (COLD). Very refreshing on a hot day. The ride up there is beutiful and most of the drivers know some spots to stop and take great pictures up in the hills. Some of the roadside venders have some of the best fruit you will ever eat. The guides are very knowlegable and entertaining.