We did the Martha Brae last year. I have to agree that it is worth doing. I enjoyed it more than Dunn's River. However, expect a BIG push for tips. The raft pilots will show you various crafts they make ("a gift") and tell you they are worth $15 or $20. You then pay what you think it is worth. We paid him $20! Another couple gave the guy $1! So be prepared! On top of that, they will want a tip.
It was an experience I will never forget!