Treasure Island is one of a small number of books that are both for children and adults. The appeal of the book for children relates to the story line: pirates, buried treasure, sea voyages to faraway places, and a boy hero. The appeal of the book for adults is in seeing a wonderful example of how events operate at many different levels. Long John Silver quickly becomes the focus for adults. What is his true nature? What will he do next? Clearly, Silver is one of the most interesting and memorable of all fictional characters.
A problem that children will have with this book is that the language is somewhat foreign to them. Some adults and children will find that the book starts slowly compared to newer novels (which often have the equivalent of a chase sequence in the first 5 pages).
My advice is to stick with the story for the first 6 chapters, and see how you are doing. By that time, the story will either have cast its spell on you, or you will be able to tell that this book is not for you.
A final reason for reading Treasure Island is because the book has been read by so many people. You will find references to the story in other literature and in conversation with others. You will also run into establishments called The Admiral Benbow Inn. It would be a shame not ot know its heritage. Also, finding someone else who likes Jim Hawkins and Long John Silver for the same reasons you do is a great shortcut to becoming better acquainted.
Personally, I found the story irresistible. I would have written a very similar book if I had the skill to do so. The plot is nicely balanced, and the characters provide an unusual perspective for what could easily have been a real potboiler with little to recommend it. The book has great charm, given its focus on pirates, which makes it compelling for me. I have now read the book 3 times, and enjoyed it more each time.
Have a great read!