I've never done a cruise ship, but have been to plenty All-Inclusive resorts around the Caribbean.
I personally like them for the convenience of having everything you need in one place, and not have to worry about cash. I have yet to stay at a Sandals resort, but the other places I've went to, the buffet food is descent, plenty of options for everyone. The A La carte restaurants are better, but you will need to do reservation ahead of time (Usually the morning of or the day before). The drinks (If you do drink) are great if they are included. They might try to give you watered down drinks but you can always ask for more.
Make sure to look into activities and what's included. A lot of times you get Snorkeling, Kayak, and some other water activities included. There might be a few that are not included and you need to pay extra (parasailing, Jet skiing, etc). There are usually entertainment at night as well, like live concerts and what not.
The one thing for me, is that after day 4-5 I start to get bored, so I usually try to do it for 3-4 nights max.
Edit: Depending where you will be in Jamaica, the water could be very active, so I recommend snorkeling.