We are getting our first cabana on our sailing next week. For us it was worth it. There are 8 of us. My parents have health issues and could not spend that much time in the sun. The cost worked out to about $50/person which would be the cost of snorkel gear and floats so the price was a wash as we were planning on renting those anyway. I'm not sure I would try for a cabana if there were only my immediate family of 3 as we would spend more time in the ocean and less time in the cabana. But with older people with health issues, the golf cart service alone will be worth it.
I think the "is it worth it" question has many factors that only you can answer. Some like the pampering, some like the privacy, some like the shade. It depends upon how you plan on spending your time on the island. If you are big ocean or excursion people, probably not right for you. If you are more relaxing in the sun (or shade) people, that may move it to the more worth it pile. Would this be a great memory for a honeymoon or other special occasion? Do you have something else you would rather spend the money on? More people to split the cost with? All of these depend upon your personal preference and budget. I don't think one is better or worse than the other, just different.