Tom Sawyer Island is a must-do if you have kids that are 6-16 yo
But as an adult without kids, I would judge that the best reasons to go are:
- You are a big Mark Twain fan
- You like to explore (caves, secret passages, bridges, etc)
- You are looking to get away from the crowds for a little while (there is a GREAT place to rest near the old Aunt Polly's)
in 1972, Tom Sawyer Island was a 'D' ticket attraction (as was the Peoplemover and the Railroad) - just to give you some equivalency
It'll take about an hour to fully explore the attraction.