I'll first point out that I haven't seen any of the other older versions of Treasure Island, nor was I familiar with the story prior to watching this.
My initial feelings are that Pirates Of The Caribbean has been put to shame for it's immature attitude towards pirates, whilst this film is probably the best representation I've seen on screen (but I haven't watched a lot of pirate themed films, to be honest). We are treated to a slightly rugged world of pirates where there are no supermodels running about, quite the opposite for the most part. The way it should be. I think the cleanest looking person in the whole film is Elijah Woods.
Eddie Izzard (Long John Silver) was excellent and perfectly cast in my opinion. I have never thought much of him as an actor before but this certainly changed my mind. Just like PotC is all about Depp, this is all about Izzard. The rest of the cast are also very very good but Izzard really stands out. Sean Gilder also made an excellent pirate, but his on screen time was pretty limited.
I can't comment on how true this is to the original story but I thought it was great entertainment with really good production values and a great overall cast. Easily the best pirate film I've seen.