I've played in scripted plot games (scripted to various degrees) but I've never played in a sandbox game.
The benefit to a scripted plot is an easy to follow storyline and everyone follows the dots to the end. In my opinion, the bad thing about a scripted plot is the feeling that the story is moving along on rails and I don't need to post because nothing my character does makes a difference.
From reading the mention of sandbox games other places, I get that liking or disliking sandbox games is a matter of personal preference, but what are some benefits and pitfalls to this type of game?